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+========================================
+:mod:`turtle` --- Turtle graphics for Tk
+========================================
+
+.. module:: turtle
+   :synopsis: Turtle graphics for Tk
+.. sectionauthor:: Gregor Lingl <gregor.lingl@aon.at>
+
+Introduction
+============
+
+Turtle graphics is a popular way for introducing programming to kids.  It was
+part of the original Logo programming language developed by Wally Feurzig and
+Seymour Papert in 1966.
+
+Imagine a robotic turtle starting at (0, 0) in the x-y plane.  Give it the
+command ``turtle.forward(15)``, and it moves (on-screen!) 15 pixels in the
+direction it is facing, drawing a line as it moves.  Give it the command
+``turtle.left(25)``, and it rotates in-place 25 degrees clockwise.
+
+By combining together these and similar commands, intricate shapes and pictures
+can easily be drawn.
+
+The :mod:`turtle` module is an extended reimplementation of the same-named
+module from the Python standard distribution up to version Python 2.5.
+
+It tries to keep the merits of the old turtle module and to be (nearly) 100%
+compatible with it.  This means in the first place to enable the learning
+programmer to use all the commands, classes and methods interactively when using
+the module from within IDLE run with the ``-n`` switch.
+
+The turtle module provides turtle graphics primitives, in both object-oriented
+and procedure-oriented ways.  Because it uses :mod:`Tkinter` for the underlying
+graphics, it needs a version of python installed with Tk support.
+
+The object-oriented interface uses essentially two+two classes:
+
+1. The :class:`TurtleScreen` class defines graphics windows as a playground for
+   the drawing turtles.  Its constructor needs a :class:`Tkinter.Canvas` or a
+   :class:`ScrolledCanvas` as argument.  It should be used when :mod:`turtle` is
+   used as part of some application.
+
+   Derived from :class:`TurtleScreen` is the subclass :class:`Screen`.  Screen
+   is implemented as sort of singleton, so there can exist only one instance of
+   Screen at a time.  It should be used when :mod:`turtle` is used as a
+   standalone tool for doing graphics.
+
+   All methods of TurtleScreen/Screen also exist as functions, i.e. as part of
+   the procedure-oriented interface.
+
+2. :class:`RawTurtle` (alias: :class:`RawPen`) defines Turtle objects which draw
+   on a :class:`TurtleScreen`.  Its constructor needs a Canvas, ScrolledCanvas
+   or TurtleScreen as argument, so the RawTurtle objects know where to draw.
+
+   Derived from RawTurtle is the subclass :class:`Turtle` (alias: :class:`Pen`),
+   which draws on "the" :class:`Screen` - instance which is automatically
+   created, if not already present.
+
+   All methods of RawTurtle/Turtle also exist as functions, i.e. part of the
+   procedure-oriented interface.
+
+The procedural interface provides functions which are derived from the methods
+of the classes :class:`Screen` and :class:`Turtle`.  They have the same names as
+the corresponding methods.  A screen object is automativally created whenever a
+function derived from a Screen method is called.  An (unnamed) turtle object is
+automatically created whenever any of the functions derived from a Turtle method
+is called.
+
+To use multiple turtles an a screen one has to use the object-oriented interface.
+
+.. note::
+   In the following documentation the argument list for functions is given.
+   Methods, of course, have the additional first argument *self* which is
+   omitted here.
+
+
+Overview over available Turtle and Screen methods
+=================================================
+
+Turtle methods
+--------------
+
+Turtle motion
+   Move and draw
+      | :func:`forward` | :func:`fd`
+      | :func:`backward` | :func:`bk` | :func:`back`
+      | :func:`right` | :func:`rt`
+      | :func:`left` | :func:`lt`
+      | :func:`goto` | :func:`setpos` | :func:`setposition`
+      | :func:`setx`
+      | :func:`sety`
+      | :func:`setheading` | :func:`seth`
+      | :func:`home`
+      | :func:`circle`
+      | :func:`dot`
+      | :func:`stamp`
+      | :func:`clearstamp`
+      | :func:`clearstamps`
+      | :func:`undo`
+      | :func:`speed`
+
+   Tell Turtle's state
+      | :func:`position` | :func:`pos`
+      | :func:`towards`
+      | :func:`xcor`
+      | :func:`ycor`
+      | :func:`heading`
+      | :func:`distance`
+
+   Setting and measurement
+      | :func:`degrees`
+      | :func:`radians`
+
+Pen control
+   Drawing state
+      | :func:`pendown` | :func:`pd` | :func:`down`
+      | :func:`penup` | :func:`pu` | :func:`up`
+      | :func:`pensize` | :func:`width`
+      | :func:`pen`
+      | :func:`isdown`
+
+   Color control
+      | :func:`color`
+      | :func:`pencolor`
+      | :func:`fillcolor`
+
+   Filling
+      | :func:`filling`
+      | :func:`begin_fill`
+      | :func:`end_fill`
+
+   More drawing control
+      | :func:`reset`
+      | :func:`clear`
+      | :func:`write`
+
+Turtle state
+   Visibility
+      | :func:`showturtle` | :func:`st`
+      | :func:`hideturtle` | :func:`ht`
+      | :func:`isvisible`
+
+   Appearance
+      | :func:`shape`
+      | :func:`resizemode`
+      | :func:`shapesize` | :func:`turtlesize`
+      | :func:`settiltangle`
+      | :func:`tiltangle`
+      | :func:`tilt`
+
+Using events
+   | :func:`onclick`
+   | :func:`onrelease`
+   | :func:`ondrag`
+
+Special Turtle methods
+   | :func:`begin_poly`
+   | :func:`end_poly`
+   | :func:`get_poly`
+   | :func:`clone`
+   | :func:`getturtle` | :func:`getpen`
+   | :func:`getscreen`
+   | :func:`setundobuffer`
+   | :func:`undobufferentries`
+
+
+Methods of TurtleScreen/Screen
+------------------------------
+
+Window control
+   | :func:`bgcolor`
+   | :func:`bgpic`
+   | :func:`clear` | :func:`clearscreen`
+   | :func:`reset` | :func:`resetscreen`
+   | :func:`screensize`
+   | :func:`setworldcoordinates`
+
+Animation control
+   | :func:`delay`
+   | :func:`tracer`
+   | :func:`update`
+
+Using screen events
+   | :func:`listen`
+   | :func:`onkey`
+   | :func:`onclick` | :func:`onscreenclick`
+   | :func:`ontimer`
+
+Settings and special methods
+   | :func:`mode`
+   | :func:`colormode`
+   | :func:`getcanvas`
+   | :func:`getshapes`
+   | :func:`register_shape` | :func:`addshape`
+   | :func:`turtles`
+   | :func:`window_height`
+   | :func:`window_width`
+
+Methods specific to Screen
+   | :func:`bye`
+   | :func:`exitonclick`
+   | :func:`setup`
+   | :func:`title`
+
+
+Methods of RawTurtle/Turtle and corresponding functions
+=======================================================
+
+Most of the examples in this section refer to a Turtle instance called
+``turtle``.
+
+Turtle motion
+-------------
+
+.. function:: forward(distance)
+              fd(distance)
+
+   :param distance: a number (integer or float)
+
+   Move the turtle forward by the specified *distance*, in the direction the
+   turtle is headed.
+
+   >>> turtle.position()
+   (0.00, 0.00)
+   >>> turtle.forward(25)
+   >>> turtle.position()
+   (25.00,0.00)
+   >>> turtle.forward(-75)
+   >>> turtle.position()
+   (-50.00,0.00)
+
+
+.. function:: back(distance)
+              bk(distance)
+              backward(distance)
+
+   :param distance: a number
+
+   Move the turtle backward by *distance*, opposite to the direction the
+   turtle is headed.  Do not change the turtle's heading.
+
+   >>> turtle.position()
+   (0.00, 0.00)
+   >>> turtle.backward(30)
+   >>> turtle.position()
+   (-30.00, 0.00)
+
+
+.. function:: right(angle)
+              rt(angle)
+
+   :param angle: a number (integer or float)
+
+   Turn turtle right by *angle* units.  (Units are by default degrees, but
+   can be set via the :func:`degrees` and :func:`radians` functions.)  Angle
+   orientation depends on the turtle mode, see :func:`mode`.
+
+   >>> turtle.heading()
+   22.0
+   >>> turtle.right(45)
+   >>> turtle.heading()
+   337.0
+
+
+.. function:: left(angle)
+              lt(angle)
+
+   :param angle: a number (integer or float)
+
+   Turn turtle left by *angle* units.  (Units are by default degrees, but
+   can be set via the :func:`degrees` and :func:`radians` functions.)  Angle
+   orientation depends on the turtle mode, see :func:`mode`.
+
+   >>> turtle.heading()
+   22.0
+   >>> turtle.left(45)
+   >>> turtle.heading()
+   67.0
+
+.. function:: goto(x, y=None)
+              setpos(x, y=None)
+              setposition(x, y=None)
+
+    :param x: a number or a pair/vector of numbers
+    :param y: a number or ``None``
+
+    If *y* is ``None``, *x* must be a pair of coordinates or a :class:`Vec2D`
+    (e.g. as returned by :func:`pos`).
+
+    Move turtle to an absolute position.  If the pen is down, draw line.  Do
+    not change the turtle's orientation.
+
+    >>> tp = turtle.pos()
+    >>> tp
+    (0.00, 0.00)
+    >>> turtle.setpos(60,30)
+    >>> turtle.pos()
+    (60.00,30.00)
+    >>> turtle.setpos((20,80))
+    >>> turtle.pos()
+    (20.00,80.00)
+    >>> turtle.setpos(tp)
+    >>> turtle.pos()
+    (0.00,0.00)
+
+
+.. function:: setx(x)
+
+   :param x: a number (integer or float)
+
+   Set the turtle's first coordinate to *x*, leave second coordinate
+   unchanged.
+
+   >>> turtle.position()
+   (0.00, 240.00)
+   >>> turtle.setx(10)
+   >>> turtle.position()
+   (10.00, 240.00)
+
+
+.. function:: sety(y)
+
+   :param y: a number (integer or float)
+
+   Set the turtle's first coordinate to *y*, leave second coordinate
+   unchanged.
+
+   >>> turtle.position()
+   (0.00, 40.00)
+   >>> turtle.sety(-10)
+   >>> turtle.position()
+   (0.00, -10.00)
+
+
+.. function:: setheading(to_angle)
+              seth(to_angle)
+
+   :param to_angle: a number (integer or float)
+
+   Set the orientation of the turtle to *to_angle*.  Here are some common
+   directions in degrees:
+
+   =================== ====================
+    standard mode           logo mode
+   =================== ====================
+      0 - east                0 - north
+     90 - north              90 - east
+    180 - west              180 - south
+    270 - south             270 - west
+   =================== ====================
+
+   >>> turtle.setheading(90)
+   >>> turtle.heading()
+   90
+
+
+.. function:: home()
+
+   Move turtle to the origin -- coordinates (0,0) -- and set its heading to
+   its start-orientation (which depends on the mode, see :func:`mode`).
+
+
+.. function:: circle(radius, extent=None, steps=None)
+
+   :param radius: a number
+   :param extent: a number (or ``None``)
+   :param steps: an integer (or ``None``)
+
+   Draw a circle with given *radius*.  The center is *radius* units left of
+   the turtle; *extent* -- an angle -- determines which part of the circle
+   is drawn.  If *extent* is not given, draw the entire circle.  If *extent*
+   is not a full circle, one endpoint of the arc is the current pen
+   position.  Draw the arc in counterclockwise direction if *radius* is
+   positive, otherwise in clockwise direction.  Finally the direction of the
+   turtle is changed by the amount of *extent*.
+
+   As the circle is approximated by an inscribed regular polygon, *steps*
+   determines the number of steps to use.  If not given, it will be
+   calculated automatically.  May be used to draw regular polygons.
+
+   >>> turtle.circle(50)
+   >>> turtle.circle(120, 180)  # draw a semicircle
+
+
+.. function:: dot(size=None, *color)
+
+   :param size: an integer >= 1 (if given)
+   :param color: a colorstring or a numeric color tuple
+
+   Draw a circular dot with diameter *size*, using *color*.  If *size* is
+   not given, the maximum of pensize+4 and 2*pensize is used.
+
+   >>> turtle.dot()
+   >>> turtle.fd(50); turtle.dot(20, "blue"); turtle.fd(50)
+
+
+.. function:: stamp()
+
+   Stamp a copy of the turtle shape onto the canvas at the current turtle
+   position.  Return a stamp_id for that stamp, which can be used to delete
+   it by calling ``clearstamp(stamp_id)``.
+
+   >>> turtle.color("blue")
+   >>> turtle.stamp()
+   13
+   >>> turtle.fd(50)
+
+
+.. function:: clearstamp(stampid)
+
+   :param stampid: an integer, must be return value of previous
+                   :func:`stamp` call
+
+   Delete stamp with given *stampid*.
+
+   >>> turtle.color("blue")
+   >>> astamp = turtle.stamp()
+   >>> turtle.fd(50)
+   >>> turtle.clearstamp(astamp)
+
+
+.. function:: clearstamps(n=None)
+
+   :param n: an integer (or ``None``)
+
+   Delete all or first/last *n* of turtle's stamps.  If *n* is None, delete
+   all stamps, if *n* > 0 delete first *n* stamps, else if *n* < 0 delete
+   last *n* stamps.
+
+   >>> for i in range(8):
+   ...     turtle.stamp(); turtle.fd(30)
+   >>> turtle.clearstamps(2)
+   >>> turtle.clearstamps(-2)
+   >>> turtle.clearstamps()
+
+
+.. function:: undo()
+
+   Undo (repeatedly) the last turtle action(s).  Number of available
+   undo actions is determined by the size of the undobuffer.
+
+   >>> for i in range(4):
+   ...     turtle.fd(50); turtle.lt(80)
+   ...
+   >>> for i in range(8):
+   ...     turtle.undo()
+
+
+.. function:: speed(speed=None)
+
+   :param speed: an integer in the range 0..10 or a speedstring (see below)
+
+   Set the turtle's speed to an integer value in the range 0..10.  If no
+   argument is given, return current speed.
+
+   If input is a number greater than 10 or smaller than 0.5, speed is set
+   to 0.  Speedstrings are mapped to speedvalues as follows:
+
+   * "fastest":  0
+   * "fast":  10
+   * "normal":  6
+   * "slow":  3
+   * "slowest":  1
+
+   Speeds from 1 to 10 enforce increasingly faster animation of line drawing
+   and turtle turning.
+
+   Attention: *speed* = 0 means that *no* animation takes
+   place. forward/back makes turtle jump and likewise left/right make the
+   turtle turn instantly.
+
+   >>> turtle.speed(3)
+
+
+Tell Turtle's state
+-------------------
+
+.. function:: position()
+              pos()
+
+   Return the turtle's current location (x,y) (as a :class:`Vec2D` vector).
+
+   >>> turtle.pos()
+   (0.00, 240.00)
+
+
+.. function:: towards(x, y=None)
+
+   :param x: a number or a pair/vector of numbers or a turtle instance
+   :param y: a number if *x* is a number, else ``None``
+
+   Return the angle between the line from turtle position to position specified
+   by (x,y), the vector or the other turtle.  This depends on the turtle's start
+   orientation which depends on the mode - "standard"/"world" or "logo").
+
+   >>> turtle.pos()
+   (10.00, 10.00)
+   >>> turtle.towards(0,0)
+   225.0
+
+
+.. function:: xcor()
+
+   Return the turtle's x coordinate.
+
+   >>> reset()
+   >>> turtle.left(60)
+   >>> turtle.forward(100)
+   >>> print turtle.xcor()
+   50.0
+
+
+.. function:: ycor()
+
+   Return the turtle's y coordinate.
+
+   >>> reset()
+   >>> turtle.left(60)
+   >>> turtle.forward(100)
+   >>> print turtle.ycor()
+   86.6025403784
+
+
+.. function:: heading()
+
+   Return the turtle's current heading (value depends on the turtle mode, see
+   :func:`mode`).
+
+   >>> turtle.left(67)
+   >>> turtle.heading()
+   67.0
+
+
+.. function:: distance(x, y=None)
+
+   :param x: a number or a pair/vector of numbers or a turtle instance
+   :param y: a number if *x* is a number, else ``None``
+
+   Return the distance from the turtle to (x,y), the given vector, or the given
+   other turtle, in turtle step units.
+
+   >>> turtle.pos()
+   (0.00, 0.00)
+   >>> turtle.distance(30,40)
+   50.0
+   >>> joe = Turtle()
+   >>> joe.forward(77)
+   >>> turtle.distance(joe)
+   77.0
+
+
+Settings for measurement
+------------------------
+
+.. function:: degrees(fullcircle=360.0)
+
+   :param fullcircle: a number
+
+   Set angle measurement units, i.e. set number of "degrees" for a full circle.
+   Default value is 360 degrees.
+
+   >>> turtle.left(90)
+   >>> turtle.heading()
+   90
+   >>> turtle.degrees(400.0)  # angle measurement in gon
+   >>> turtle.heading()
+   100
+
+
+.. function:: radians()
+
+   Set the angle measurement units to radians.  Equivalent to
+   ``degrees(2*math.pi)``.
+
+   >>> turtle.heading()
+   90
+   >>> turtle.radians()
+   >>> turtle.heading()
+   1.5707963267948966
+
+
+Pen control
+-----------
+
+Drawing state
+~~~~~~~~~~~~~
+
+.. function:: pendown()
+              pd()
+              down()
+
+   Pull the pen down -- drawing when moving.
+
+
+.. function:: penup()
+              pu()
+              up()
+
+   Pull the pen up -- no drawing when moving.
+
+
+.. function:: pensize(width=None)
+              width(width=None)
+
+   :param width: a positive number
+
+   Set the line thickness to *width* or return it.  If resizemode is set to
+   "auto" and turtleshape is a polygon, that polygon is drawn with the same line
+   thickness.  If no argument is given, the current pensize is returned.
+
+   >>> turtle.pensize()
+   1
+   >>> turtle.pensize(10)   # from here on lines of width 10 are drawn
+
+
+.. function:: pen(pen=None, **pendict)
+
+   :param pen: a dictionary with some or all of the below listed keys
+   :param pendict: one or more keyword-arguments with the below listed keys as keywords
+
+   Return or set the pen's attributes in a "pen-dictionary" with the following
+   key/value pairs:
+
+   * "shown": True/False
+   * "pendown": True/False
+   * "pencolor": color-string or color-tuple
+   * "fillcolor": color-string or color-tuple
+   * "pensize": positive number
+   * "speed": number in range 0..10
+   * "resizemode": "auto" or "user" or "noresize"
+   * "stretchfactor": (positive number, positive number)
+   * "outline": positive number
+   * "tilt": number
+
+   This dicionary can be used as argument for a subsequent call to :func:`pen`
+   to restore the former pen-state.  Moreover one or more of these attributes
+   can be provided as keyword-arguments.  This can be used to set several pen
+   attributes in one statement.
+
+   >>> turtle.pen(fillcolor="black", pencolor="red", pensize=10)
+   >>> turtle.pen()
+   {'pensize': 10, 'shown': True, 'resizemode': 'auto', 'outline': 1,
+   'pencolor': 'red', 'pendown': True, 'fillcolor': 'black',
+   'stretchfactor': (1,1), 'speed': 3}
+   >>> penstate=turtle.pen()
+   >>> turtle.color("yellow","")
+   >>> turtle.penup()
+   >>> turtle.pen()
+   {'pensize': 10, 'shown': True, 'resizemode': 'auto', 'outline': 1,
+   'pencolor': 'yellow', 'pendown': False, 'fillcolor': '',
+   'stretchfactor': (1,1), 'speed': 3}
+   >>> p.pen(penstate, fillcolor="green")
+   >>> p.pen()
+   {'pensize': 10, 'shown': True, 'resizemode': 'auto', 'outline': 1,
+   'pencolor': 'red', 'pendown': True, 'fillcolor': 'green',
+   'stretchfactor': (1,1), 'speed': 3}
+
+
+.. function:: isdown()
+
+   Return ``True`` if pen is down, ``False`` if it's up.
+
+   >>> turtle.penup()
+   >>> turtle.isdown()
+   False
+   >>> turtle.pendown()
+   >>> turtle.isdown()
+   True
+
+
+Color control
+~~~~~~~~~~~~~
+
+.. function:: pencolor(*args)
+
+   Return or set the pencolor.
+
+   Four input formats are allowed:
+
+   ``pencolor()``
+      Return the current pencolor as color specification string, possibly in
+      hex-number format (see example).  May be used as input to another
+      color/pencolor/fillcolor call.
+
+   ``pencolor(colorstring)``
+      Set pencolor to *colorstring*, which is a Tk color specification string,
+      such as ``"red"``, ``"yellow"``, or ``"#33cc8c"``.
+
+   ``pencolor((r, g, b))``
+      Set pencolor to the RGB color represented by the tuple of *r*, *g*, and
+      *b*.  Each of *r*, *g*, and *b* must be in the range 0..colormode, where
+      colormode is either 1.0 or 255 (see :func:`colormode`).
+
+   ``pencolor(r, g, b)``
+      Set pencolor to the RGB color represented by *r*, *g*, and *b*.  Each of
+      *r*, *g*, and *b* must be in the range 0..colormode.
+
+    If turtleshape is a polygon, the outline of that polygon is drawn with the
+    newly set pencolor.
+
+    >>> turtle.pencolor("brown")
+    >>> tup = (0.2, 0.8, 0.55)
+    >>> turtle.pencolor(tup)
+    >>> turtle.pencolor()
+    "#33cc8c"
+
+
+.. function:: fillcolor(*args)
+
+   Return or set the fillcolor.
+
+   Four input formats are allowed:
+
+   ``fillcolor()``
+      Return the current fillcolor as color specification string, possibly in
+      hex-number format (see example).  May be used as input to another
+      color/pencolor/fillcolor call.
+
+   ``fillcolor(colorstring)``
+      Set fillcolor to *colorstring*, which is a Tk color specification string,
+      such as ``"red"``, ``"yellow"``, or ``"#33cc8c"``.
+
+   ``fillcolor((r, g, b))``
+      Set fillcolor to the RGB color represented by the tuple of *r*, *g*, and
+      *b*.  Each of *r*, *g*, and *b* must be in the range 0..colormode, where
+      colormode is either 1.0 or 255 (see :func:`colormode`).
+
+   ``fillcolor(r, g, b)``
+      Set fillcolor to the RGB color represented by *r*, *g*, and *b*.  Each of
+      *r*, *g*, and *b* must be in the range 0..colormode.
+
+    If turtleshape is a polygon, the interior of that polygon is drawn
+    with the newly set fillcolor.
+
+    >>> turtle.fillcolor("violet")
+    >>> col = turtle.pencolor()
+    >>> turtle.fillcolor(col)
+    >>> turtle.fillcolor(0, .5, 0)
+
+
+.. function:: color(*args)
+
+   Return or set pencolor and fillcolor.
+
+   Several input formats are allowed.  They use 0 to 3 arguments as
+   follows:
+
+   ``color()``
+      Return the current pencolor and the current fillcolor as a pair of color
+      specification strings as returned by :func:`pencolor` and
+      :func:`fillcolor`.
+
+   ``color(colorstring)``, ``color((r,g,b))``, ``color(r,g,b)``
+      Inputs as in :func:`pencolor`, set both, fillcolor and pencolor, to the
+      given value.
+
+   ``color(colorstring1, colorstring2)``, ``color((r1,g1,b1), (r2,g2,b2))``
+      Equivalent to ``pencolor(colorstring1)`` and ``fillcolor(colorstring2)``
+      and analogously if the other input format is used.
+
+    If turtleshape is a polygon, outline and interior of that polygon is drawn
+    with the newly set colors.
+
+    >>> turtle.color("red", "green")
+    >>> turtle.color()
+    ("red", "green")
+    >>> colormode(255)
+    >>> color((40, 80, 120), (160, 200, 240))
+    >>> color()
+    ("#285078", "#a0c8f0")
+
+
+See also: Screen method :func:`colormode`.
+
+
+Filling
+~~~~~~~
+
+.. function:: filling()
+
+   Return fillstate (``True`` if filling, ``False`` else).
+
+   >>> turtle.begin_fill()
+   >>> if turtle.filling():
+   ...    turtle.pensize(5)
+   else:
+   ...    turtle.pensize(3)
+
+
+.. function:: begin_fill()
+
+   To be called just before drawing a shape to be filled.
+
+   >>> turtle.color("black", "red")
+   >>> turtle.begin_fill()
+   >>> turtle.circle(60)
+   >>> turtle.end_fill()
+
+
+.. function:: end_fill()
+
+   Fill the shape drawn after the last call to :func:`begin_fill`.
+
+
+More drawing control
+~~~~~~~~~~~~~~~~~~~~
+
+.. function:: reset()
+
+   Delete the turtle's drawings from the screen, re-center the turtle and set
+   variables to the default values.
+
+   >>> turtle.position()
+   (0.00,-22.00)
+   >>> turtle.heading()
+   100.0
+   >>> turtle.reset()
+   >>> turtle.position()
+   (0.00,0.00)
+   >>> turtle.heading()
+   0.0
+
+
+.. function:: clear()
+
+   Delete the turtle's drawings from the screen.  Do not move turtle.  State and
+   position of the turtle as well as drawings of other turtles are not affected.
+
+
+.. function:: write(arg, move=False, align="left", font=("Arial", 8, "normal"))
+
+   :param arg: object to be written to the TurtleScreen
+   :param move: True/False
+   :param align: one of the strings "left", "center" or right"
+   :param font: a triple (fontname, fontsize, fonttype)
+
+   Write text - the string representation of *arg* - at the current turtle
+   position according to *align* ("left", "center" or right") and with the given
+   font.  If *move* is True, the pen is moved to the bottom-right corner of the
+   text.  By default, *move* is False.
+
+   >>> turtle.write("Home = ", True, align="center")
+   >>> turtle.write((0,0), True)
+
+
+Turtle state
+------------
+
+Visibility
+~~~~~~~~~~
+
+.. function:: showturtle()
+              st()
+
+   Make the turtle visible.
+
+   >>> turtle.hideturtle()
+   >>> turtle.showturtle()
+
+
+.. function:: hideturtle()
+              ht()
+
+   Make the turtle invisible.  It's a good idea to do this while you're in the
+   middle of doing some complex drawing, because hiding the turtle speeds up the
+   drawing observably.
+
+   >>> turtle.hideturtle()
+
+
+.. function:: isvisible()
+
+   Return True if the Turtle is shown, False if it's hidden.
+
+   >>> turtle.hideturtle()
+   >>> print turtle.isvisible():
+   False
+
+
+Appearance
+~~~~~~~~~~
+
+.. function:: shape(name=None)
+
+   :param name: a string which is a valid shapename
+
+   Set turtle shape to shape with given *name* or, if name is not given, return
+   name of current shape.  Shape with *name* must exist in the TurtleScreen's
+   shape dictionary.  Initially there are the following polygon shapes: "arrow",
+   "turtle", "circle", "square", "triangle", "classic".  To learn about how to
+   deal with shapes see Screen method :func:`register_shape`.
+
+   >>> turtle.shape()
+   "arrow"
+   >>> turtle.shape("turtle")
+   >>> turtle.shape()
+   "turtle"
+
+
+.. function:: resizemode(rmode=None)
+
+   :param rmode: one of the strings "auto", "user", "noresize"
+
+   Set resizemode to one of the values: "auto", "user", "noresize".  If *rmode*
+   is not given, return current resizemode.  Different resizemodes have the
+   following effects:
+
+   - "auto": adapts the appearance of the turtle corresponding to the value of pensize.
+   - "user": adapts the appearance of the turtle according to the values of
+     stretchfactor and outlinewidth (outline), which are set by
+     :func:`shapesize`.
+   - "noresize": no adaption of the turtle's appearance takes place.
+
+   resizemode("user") is called by :func:`shapesize` when used with arguments.
+
+   >>> turtle.resizemode("noresize")
+   >>> turtle.resizemode()
+   "noresize"
+
+
+.. function:: shapesize(stretch_wid=None, stretch_len=None, outline=None)
+
+   :param stretch_wid: positive number
+   :param stretch_len: positive number
+   :param outline: positive number
+
+   Return or set the pen's attributes x/y-stretchfactors and/or outline.  Set
+   resizemode to "user".  If and only if resizemode is set to "user", the turtle
+   will be displayed stretched according to its stretchfactors: *stretch_wid* is
+   stretchfactor perpendicular to its orientation, *stretch_len* is
+   stretchfactor in direction of its orientation, *outline* determines the width
+   of the shapes's outline.
+
+   >>> turtle.resizemode("user")
+   >>> turtle.shapesize(5, 5, 12)
+   >>> turtle.shapesize(outline=8)
+
+
+.. function:: tilt(angle)
+
+   :param angle: a number
+
+   Rotate the turtleshape by *angle* from its current tilt-angle, but do *not*
+   change the turtle's heading (direction of movement).
+
+   >>> turtle.shape("circle")
+   >>> turtle.shapesize(5,2)
+   >>> turtle.tilt(30)
+   >>> turtle.fd(50)
+   >>> turtle.tilt(30)
+   >>> turtle.fd(50)
+
+
+.. function:: settiltangle(angle)
+
+   :param angle: a number
+
+   Rotate the turtleshape to point in the direction specified by *angle*,
+   regardless of its current tilt-angle.  *Do not* change the turtle's heading
+   (direction of movement).
+
+   >>> turtle.shape("circle")
+   >>> turtle.shapesize(5,2)
+   >>> turtle.settiltangle(45)
+   >>> stamp()
+   >>> turtle.fd(50)
+   >>> turtle.settiltangle(-45)
+   >>> stamp()
+   >>> turtle.fd(50)
+
+
+.. function:: tiltangle()
+
+   Return the current tilt-angle, i.e. the angle between the orientation of the
+   turtleshape and the heading of the turtle (its direction of movement).
+
+   >>> turtle.shape("circle")
+   >>> turtle.shapesize(5,2)
+   >>> turtle.tilt(45)
+   >>> turtle.tiltangle()
+   45
+
+
+Using events
+------------
+
+.. function:: onclick(fun, btn=1, add=None)
+
+   :param fun: a function with two arguments which will be called with the
+               coordinates of the clicked point on the canvas
+   :param num: number of the mouse-button, defaults to 1 (left mouse button)
+   :param add: ``True`` or ``False`` -- if ``True``, a new binding will be
+               added, otherwise it will replace a former binding
+
+   Bind *fun* to mouse-click events on this turtle.  If *fun* is ``None``,
+   existing bindings are removed.  Example for the anonymous turtle, i.e. the
+   procedural way:
+
+   >>> def turn(x, y):
+   ...     left(180)
+   ...
+   >>> onclick(turn)  # Now clicking into the turtle will turn it.
+   >>> onclick(None)  # event-binding will be removed
+
+
+.. function:: onrelease(fun, btn=1, add=None)
+
+   :param fun: a function with two arguments which will be called with the
+               coordinates of the clicked point on the canvas
+   :param num: number of the mouse-button, defaults to 1 (left mouse button)
+   :param add: ``True`` or ``False`` -- if ``True``, a new binding will be
+               added, otherwise it will replace a former binding
+
+   Bind *fun* to mouse-button-release events on this turtle.  If *fun* is
+   ``None``, existing bindings are removed.
+
+   >>> class MyTurtle(Turtle):
+   ...     def glow(self,x,y):
+   ...         self.fillcolor("red")
+   ...     def unglow(self,x,y):
+   ...         self.fillcolor("")
+   ...
+   >>> turtle = MyTurtle()
+   >>> turtle.onclick(turtle.glow)     # clicking on turtle turns fillcolor red,
+   >>> turtle.onrelease(turtle.unglow) # releasing turns it to transparent.
+
+
+.. function:: ondrag(fun, btn=1, add=None)
+
+   :param fun: a function with two arguments which will be called with the
+               coordinates of the clicked point on the canvas
+   :param num: number of the mouse-button, defaults to 1 (left mouse button)
+   :param add: ``True`` or ``False`` -- if ``True``, a new binding will be
+               added, otherwise it will replace a former binding
+
+   Bind *fun* to mouse-move events on this turtle.  If *fun* is ``None``,
+   existing bindings are removed.
+
+   Remark: Every sequence of mouse-move-events on a turtle is preceded by a
+   mouse-click event on that turtle.
+
+   >>> turtle.ondrag(turtle.goto)
+   # Subsequently, clicking and dragging the Turtle will move it across
+   # the screen thereby producing handdrawings (if pen is down).
+
+
+Special Turtle methods
+----------------------
+
+.. function:: begin_poly()
+
+   Start recording the vertices of a polygon.  Current turtle position is first
+   vertex of polygon.
+
+
+.. function:: end_poly()
+
+   Stop recording the vertices of a polygon.  Current turtle position is last
+   vertex of polygon.  This will be connected with the first vertex.
+
+
+.. function:: get_poly()
+
+   Return the last recorded polygon.
+
+   >>> p = turtle.get_poly()
+   >>> turtle.register_shape("myFavouriteShape", p)
+
+
+.. function:: clone()
+
+   Create and return a clone of the turtle with same position, heading and
+   turtle properties.
+
+   >>> mick = Turtle()
+   >>> joe = mick.clone()
+
+
+.. function:: getturtle()
+
+   Return the Turtle object itself.  Only reasonable use: as a function to
+   return the "anonymous turtle":
+
+   >>> pet = getturtle()
+   >>> pet.fd(50)
+   >>> pet
+   <turtle.Turtle object at 0x01417350>
+   >>> turtles()
+   [<turtle.Turtle object at 0x01417350>]
+
+
+.. function:: getscreen()
+
+   Return the :class:`TurtleScreen` object the turtle is drawing on.
+   TurtleScreen methods can then be called for that object.
+
+   >>> ts = turtle.getscreen()
+   >>> ts
+   <turtle.Screen object at 0x01417710>
+   >>> ts.bgcolor("pink")
+
+
+.. function:: setundobuffer(size)
+
+   :param size: an integer or ``None``
+
+   Set or disable undobuffer.  If *size* is an integer an empty undobuffer of
+   given size is installed.  *size* gives the maximum number of turtle actions
+   that can be undone by the :func:`undo` method/function.  If *size* is
+   ``None``, the undobuffer is disabled.
+
+   >>> turtle.setundobuffer(42)
+
+
+.. function:: undobufferentries()
+
+   Return number of entries in the undobuffer.
+
+   >>> while undobufferentries():
+   ...     undo()
+
+
+.. _compoundshapes:
+
+Excursus about the use of compound shapes
+-----------------------------------------
+
+To use compound turtle shapes, which consist of several polygons of different
+color, you must use the helper class :class:`Shape` explicitly as described
+below:
+
+1. Create an empty Shape object of type "compound".
+2. Add as many components to this object as desired, using the
+   :meth:`addcomponent` method.
+
+   For example:
+
+   >>> s = Shape("compound")
+   >>> poly1 = ((0,0),(10,-5),(0,10),(-10,-5))
+   >>> s.addcomponent(poly1, "red", "blue")
+   >>> poly2 = ((0,0),(10,-5),(-10,-5))
+   >>> s.addcomponent(poly2, "blue", "red")
+
+3. Now add the Shape to the Screen's shapelist and use it:
+
+   >>> register_shape("myshape", s)
+   >>> shape("myshape")
+
+
+.. note::
+
+   The :class:`Shape` class is used internally by the :func:`register_shape`
+   method in different ways.  The application programmer has to deal with the
+   Shape class *only* when using compound shapes like shown above!
+
+
+Methods of TurtleScreen/Screen and corresponding functions
+==========================================================
+
+Most of the examples in this section refer to a TurtleScreen instance called
+``screen``.
+
+
+Window control
+--------------
+
+.. function:: bgcolor(*args)
+
+   :param args: a color string or three numbers in the range 0..colormode or a
+                3-tuple of such numbers
+
+   Set or return background color of the TurtleScreen.
+
+   >>> screen.bgcolor("orange")
+   >>> screen.bgcolor()
+   "orange"
+   >>> screen.bgcolor(0.5,0,0.5)
+   >>> screen.bgcolor()
+   "#800080"
+
+
+.. function:: bgpic(picname=None)
+
+   :param picname: a string, name of a gif-file or ``"nopic"``, or ``None``
+
+   Set background image or return name of current backgroundimage.  If *picname*
+   is a filename, set the corresponding image as background.  If *picname* is
+   ``"nopic"``, delete background image, if present.  If *picname* is ``None``,
+   return the filename of the current backgroundimage.
+
+   >>> screen.bgpic()
+   "nopic"
+   >>> screen.bgpic("landscape.gif")
+   >>> screen.bgpic()
+   "landscape.gif"
+
+
+.. function:: clear()
+              clearscreen()
+
+   Delete all drawings and all turtles from the TurtleScreen.  Reset the now
+   empty TurtleScreen to its initial state: white background, no background
+   image, no event bindings and tracing on.
+
+   .. note::
+      This TurtleScreen method is available as a global function only under the
+      name ``clearscreen``.  The global function ``clear`` is another one
+      derived from the Turtle method ``clear``.
+
+
+.. function:: reset()
+              resetscreen()
+
+   Reset all Turtles on the Screen to their initial state.
+
+   .. note::
+      This TurtleScreen method is available as a global function only under the
+      name ``resetscreen``.  The global function ``reset`` is another one
+      derived from the Turtle method ``reset``.
+
+
+.. function:: screensize(canvwidth=None, canvheight=None, bg=None)
+
+   :param canvwidth: positive integer, new width of canvas in pixels
+   :param canvheight: positive integer, new height of canvas in pixels
+   :param bg: colorstring or color-tupel, new background color
+
+   If no arguments are given, return current (canvaswidth, canvasheight).  Else
+   resize the canvas the turtles are drawing on.  Do not alter the drawing
+   window.  To observe hidden parts of the canvas, use the scrollbars. With this
+   method, one can make visible those parts of a drawing which were outside the
+   canvas before.
+
+      >>> turtle.screensize(2000,1500)
+      # e.g. to search for an erroneously escaped turtle ;-)
+
+
+.. function:: setworldcoordinates(llx, lly, urx, ury)
+
+   :param llx: a number, x-coordinate of lower left corner of canvas
+   :param lly: a number, y-coordinate of lower left corner of canvas
+   :param urx: a number, x-coordinate of upper right corner of canvas
+   :param ury: a number, y-coordinate of upper right corner of canvas
+
+   Set up user-defined coordinate system and switch to mode "world" if
+   necessary.  This performs a ``screen.reset()``.  If mode "world" is already
+   active, all drawings are redrawn according to the new coordinates.
+
+   **ATTENTION**: in user-defined coordinate systems angles may appear
+   distorted.
+
+   >>> screen.reset()
+   >>> screen.setworldcoordinates(-50,-7.5,50,7.5)
+   >>> for _ in range(72):
+   ...     left(10)
+   ...
+   >>> for _ in range(8):
+   ...     left(45); fd(2)   # a regular octagon
+
+
+Animation control
+-----------------
+
+.. function:: delay(delay=None)
+
+   :param delay: positive integer
+
+   Set or return the drawing *delay* in milliseconds.  (This is approximately
+   the time interval between two consecutive canvas updates.)  The longer the
+   drawing delay, the slower the animation.
+
+   Optional argument:
+
+   >>> screen.delay(15)
+   >>> screen.delay()
+   15
+
+
+.. function:: tracer(n=None, delay=None)
+
+   :param n: nonnegative integer
+   :param delay: nonnegative integer
+
+   Turn turtle animation on/off and set delay for update drawings.  If *n* is
+   given, only each n-th regular screen update is really performed.  (Can be
+   used to accelerate the drawing of complex graphics.)  Second argument sets
+   delay value (see :func:`delay`).
+
+   >>> screen.tracer(8, 25)
+   >>> dist = 2
+   >>> for i in range(200):
+   ...     fd(dist)
+   ...     rt(90)
+   ...     dist += 2
+
+
+.. function:: update()
+
+   Perform a TurtleScreen update. To be used when tracer is turned off.
+
+See also the RawTurtle/Turtle method :func:`speed`.
+
+
+Using screen events
+-------------------
+
+.. function:: listen(xdummy=None, ydummy=None)
+
+   Set focus on TurtleScreen (in order to collect key-events).  Dummy arguments
+   are provided in order to be able to pass :func:`listen` to the onclick method.
+
+
+.. function:: onkey(fun, key)
+
+   :param fun: a function with no arguments or ``None``
+   :param key: a string: key (e.g. "a") or key-symbol (e.g. "space")
+
+   Bind *fun* to key-release event of key.  If *fun* is ``None``, event bindings
+   are removed. Remark: in order to be able to register key-events, TurtleScreen
+   must have the focus. (See method :func:`listen`.)
+
+   >>> def f():
+   ...     fd(50)
+   ...     lt(60)
+   ...
+   >>> screen.onkey(f, "Up")
+   >>> screen.listen()
+
+
+.. function:: onclick(fun, btn=1, add=None)
+              onscreenclick(fun, btn=1, add=None)
+
+   :param fun: a function with two arguments which will be called with the
+               coordinates of the clicked point on the canvas
+   :param num: number of the mouse-button, defaults to 1 (left mouse button)
+   :param add: ``True`` or ``False`` -- if ``True``, a new binding will be
+               added, otherwise it will replace a former binding
+
+   Bind *fun* to mouse-click events on this screen.  If *fun* is ``None``,
+   existing bindings are removed.
+
+   Example for a TurtleScreen instance named ``screen`` and a Turtle instance
+   named turtle:
+
+   >>> screen.onclick(turtle.goto)
+   # Subsequently clicking into the TurtleScreen will
+   # make the turtle move to the clicked point.
+   >>> screen.onclick(None)  # remove event binding again
+
+   .. note::
+      This TurtleScreen method is available as a global function only under the
+      name ``onscreenclick``.  The global function ``onclick`` is another one
+      derived from the Turtle method ``onclick``.
+
+
+.. function:: ontimer(fun, t=0)
+
+   :param fun: a function with no arguments
+   :param t: a number >= 0
+
+   Install a timer that calls *fun* after *t* milliseconds.
+
+   >>> running = True
+   >>> def f():
+           if running:
+               fd(50)
+               lt(60)
+               screen.ontimer(f, 250)
+   >>> f()   ### makes the turtle marching around
+   >>> running = False
+
+
+Settings and special methods
+----------------------------
+
+.. function:: mode(mode=None)
+
+   :param mode: one of the strings "standard", "logo" or "world"
+
+   Set turtle mode ("standard", "logo" or "world") and perform reset.  If mode
+   is not given, current mode is returned.
+
+   Mode "standard" is compatible with old :mod:`turtle`.  Mode "logo" is
+   compatible with most Logo turtle graphics.  Mode "world" uses user-defined
+   "world coordinates". **Attention**: in this mode angles appear distorted if
+   ``x/y`` unit-ratio doesn't equal 1.
+
+   ============ ========================= ===================
+       Mode      Initial turtle heading     positive angles
+   ============ ========================= ===================
+    "standard"    to the right (east)       counterclockwise
+      "logo"        upward    (north)         clockwise
+   ============ ========================= ===================
+
+   >>> mode("logo")   # resets turtle heading to north
+   >>> mode()
+   "logo"
+
+
+.. function:: colormode(cmode=None)
+
+   :param cmode: one of the values 1.0 or 255
+
+   Return the colormode or set it to 1.0 or 255.  Subsequently *r*, *g*, *b*
+   values of color triples have to be in the range 0..\ *cmode*.
+
+   >>> screen.colormode()
+   1.0
+   >>> screen.colormode(255)
+   >>> turtle.pencolor(240,160,80)
+
+
+.. function:: getcanvas()
+
+   Return the Canvas of this TurtleScreen.  Useful for insiders who know what to
+   do with a Tkinter Canvas.
+
+   >>> cv = screen.getcanvas()
+   >>> cv
+   <turtle.ScrolledCanvas instance at 0x010742D8>
+
+
+.. function:: getshapes()
+
+   Return a list of names of all currently available turtle shapes.
+
+   >>> screen.getshapes()
+   ["arrow", "blank", "circle", ..., "turtle"]
+
+
+.. function:: register_shape(name, shape=None)
+              addshape(name, shape=None)
+
+   There are three different ways to call this function:
+
+   (1) *name* is the name of a gif-file and *shape* is ``None``: Install the
+       corresponding image shape.
+
+       .. note::
+          Image shapes *do not* rotate when turning the turtle, so they do not
+          display the heading of the turtle!
+
+   (2) *name* is an arbitrary string and *shape* is a tuple of pairs of
+       coordinates: Install the corresponding polygon shape.
+
+   (3) *name* is an arbitrary string and shape is a (compound) :class:`Shape`
+       object: Install the corresponding compound shape.
+
+   Add a turtle shape to TurtleScreen's shapelist.  Only thusly registered
+   shapes can be used by issuing the command ``shape(shapename)``.
+
+   >>> screen.register_shape("turtle.gif")
+   >>> screen.register_shape("triangle", ((5,-3), (0,5), (-5,-3)))
+
+
+.. function:: turtles()
+
+   Return the list of turtles on the screen.
+
+   >>> for turtle in screen.turtles()
+   ...     turtle.color("red")
+
+
+.. function:: window_height()
+
+   Return the height of the turtle window.
+
+   >>> screen.window_height()
+   480
+
+
+.. function:: window_width()
+
+   Return the width of the turtle window.
+
+   >>> screen.window_width()
+   640
+
+
+.. _screenspecific:
+
+Methods specific to Screen, not inherited from TurtleScreen
+-----------------------------------------------------------
+
+.. function:: bye()
+
+   Shut the turtlegraphics window.
+
+
+.. function:: exitonclick()
+
+   Bind bye() method to mouse clicks on the Screen.
+
+
+   If the value "using_IDLE" in the configuration dictionary is ``False``
+   (default value), also enter mainloop.  Remark: If IDLE with the ``-n`` switch
+   (no subprocess) is used, this value should be set to ``True`` in
+   :file:`turtle.cfg`.  In this case IDLE's own mainloop is active also for the
+   client script.
+
+
+.. function:: setup(width=_CFG["width"], height=_CFG["height"], startx=_CFG["leftright"], starty=_CFG["topbottom"])
+
+   Set the size and position of the main window.  Default values of arguments
+   are stored in the configuration dicionary and can be changed via a
+   :file:`turtle.cfg` file.
+
+   :param width: if an integer, a size in pixels, if a float, a fraction of the
+                 screen; default is 50% of screen
+   :param height: if an integer, the height in pixels, if a float, a fraction of
+                  the screen; default is 75% of screen
+   :param startx: if positive, starting position in pixels from the left
+                  edge of the screen, if negative from the right edge, if None,
+                  center window horizontally
+   :param startx: if positive, starting position in pixels from the top
+                  edge of the screen, if negative from the bottom edge, if None,
+                  center window vertically
+
+   >>> screen.setup (width=200, height=200, startx=0, starty=0)
+   # sets window to 200x200 pixels, in upper left of screen
+   >>> screen.setup(width=.75, height=0.5, startx=None, starty=None)
+   # sets window to 75% of screen by 50% of screen and centers
+
+
+.. function:: title(titlestring)
+
+   :param titlestring: a string that is shown in the titlebar of the turtle
+                       graphics window
+
+   Set title of turtle window to *titlestring*.
+
+   >>> screen.title("Welcome to the turtle zoo!")
+
+
+The public classes of the module :mod:`turtle`
+==============================================
+
+
+.. class:: RawTurtle(canvas)
+           RawPen(canvas)
+
+   :param canvas: a :class:`Tkinter.Canvas`, a :class:`ScrolledCanvas` or a
+                  :class:`TurtleScreen`
+
+    Create a turtle.  The turtle has all methods described above as "methods of
+    Turtle/RawTurtle".
+
+
+.. class:: Turtle()
+
+    Subclass of RawTurtle, has the same interface but draws on a default
+    :class:`Screen` object created automatically when needed for the first time.
+
+
+.. class:: TurtleScreen(cv)
+
+   :param cv: a :class:`Tkinter.Canvas`
+
+   Provides screen oriented methods like :func:`setbg` etc. that are described
+   above.
+
+.. class:: Screen()
+
+   Subclass of TurtleScreen, with :ref:`four methods added <screenspecific>`.
+
+   
+.. class:: ScrolledCavas(master)
+
+   :param master: some Tkinter widget to contain the ScrolledCanvas, i.e.
+      a Tkinter-canvas with scrollbars added
+
+   Used by class Screen, which thus automatically provides a ScrolledCanvas as
+   playground for the turtles.
+
+.. class:: Shape(type_, data)
+
+   :param type\_: one of the strings "polygon", "image", "compound"
+
+   Data structure modeling shapes.  The pair ``(type_, data)`` must follow this
+   specification:
+
+
+   =========== ===========
+   *type_*     *data*
+   =========== ===========
+   "polygon"   a polygon-tuple, i.e. a tuple of pairs of coordinates
+   "image"     an image  (in this form only used internally!)
+   "compound"  ``None`` (a compund shape has to be constructed using the
+               :meth:`addcomponent` method)
+   =========== ===========
+                
+   .. method:: addcomponent(poly, fill, outline=None)
+
+      :param poly: a polygon, i.e. a tuple of pairs of numbers
+      :param fill: a color the *poly* will be filled with
+      :param outline: a color for the poly's outline (if given)
+     
+      Example:
+
+      >>> poly = ((0,0),(10,-5),(0,10),(-10,-5))
+      >>> s = Shape("compound")
+      >>> s.addcomponent(poly, "red", "blue")
+      # .. add more components and then use register_shape()
+
+      See :ref:`compoundshapes`.
+
+
+.. class:: Vec2D(x, y)
+
+   A two-dimensional vector class, used as a helper class for implementing
+   turtle graphics.  May be useful for turtle graphics programs too.  Derived
+   from tuple, so a vector is a tuple!
+
+   Provides (for *a*, *b* vectors, *k* number):
+
+   * ``a + b`` vector addition
+   * ``a - b`` vector subtraction
+   * ``a * b`` inner product
+   * ``k * a`` and ``a * k`` multiplication with scalar
+   * ``abs(a)`` absolute value of a
+   * ``a.rotate(angle)`` rotation
+
+
+Help and configuration
+======================
+
+How to use help
+---------------
+
+The public methods of the Screen and Turtle classes are documented extensively
+via docstrings.  So these can be used as online-help via the Python help
+facilities:
+
+- When using IDLE, tooltips show the signatures and first lines of the
+  docstrings of typed in function-/method calls.
+
+- Calling :func:`help` on methods or functions displays the docstrings::
+
+     >>> help(Screen.bgcolor)
+     Help on method bgcolor in module turtle:
+    
+     bgcolor(self, *args) unbound turtle.Screen method
+         Set or return backgroundcolor of the TurtleScreen.
+    
+         Arguments (if given): a color string or three numbers
+         in the range 0..colormode or a 3-tuple of such numbers.
+    
+    
+           >>> screen.bgcolor("orange")
+           >>> screen.bgcolor()
+           "orange"
+           >>> screen.bgcolor(0.5,0,0.5)
+           >>> screen.bgcolor()
+           "#800080"
+    
+     >>> help(Turtle.penup)
+     Help on method penup in module turtle:
+    
+     penup(self) unbound turtle.Turtle method
+         Pull the pen up -- no drawing when moving.
+    
+         Aliases: penup | pu | up
+    
+         No argument
+    
+         >>> turtle.penup()
+
+- The docstrings of the functions which are derived from methods have a modified
+  form::
+
+     >>> help(bgcolor)
+     Help on function bgcolor in module turtle:
+    
+     bgcolor(*args)
+         Set or return backgroundcolor of the TurtleScreen.
+    
+         Arguments (if given): a color string or three numbers
+         in the range 0..colormode or a 3-tuple of such numbers.
+    
+         Example::
+    
+           >>> bgcolor("orange")
+           >>> bgcolor()
+           "orange"
+           >>> bgcolor(0.5,0,0.5)
+           >>> bgcolor()
+           "#800080"
+    
+     >>> help(penup)
+     Help on function penup in module turtle:
+    
+     penup()
+         Pull the pen up -- no drawing when moving.
+    
+         Aliases: penup | pu | up
+    
+         No argument
+    
+         Example:
+         >>> penup()
+
+These modified docstrings are created automatically together with the function
+definitions that are derived from the methods at import time.
+
+
+Translation of docstrings into different languages
+--------------------------------------------------
+
+There is a utility to create a dictionary the keys of which are the method names
+and the values of which are the docstrings of the public methods of the classes
+Screen and Turtle.
+
+.. function:: write_docstringdict(filename="turtle_docstringdict")
+
+   :param filename: a string, used as filename
+
+   Create and write docstring-dictionary to a Python script with the given
+   filename.  This function has to be called explicitly (it is not used by the
+   turtle graphics classes).  The docstring dictionary will be written to the
+   Python script :file:`{filename}.py`.  It is intended to serve as a template
+   for translation of the docstrings into different languages.
+
+If you (or your students) want to use :mod:`turtle` with online help in your
+native language, you have to translate the docstrings and save the resulting
+file as e.g. :file:`turtle_docstringdict_german.py`.
+
+If you have an appropriate entry in your :file:`turtle.cfg` file this dictionary
+will be read in at import time and will replace the original English docstrings.
+
+At the time of this writing there are docstring dictionaries in German and in
+Italian.  (Requests please to glingl@aon.at.)
+
+
+
+How to configure Screen and Turtles
+-----------------------------------
+
+The built-in default configuration mimics the appearance and behaviour of the
+old turtle module in order to retain best possible compatibility with it.
+
+If you want to use a different configuration which better reflects the features
+of this module or which better fits to your needs, e.g. for use in a classroom,
+you can prepare a configuration file ``turtle.cfg`` which will be read at import
+time and modify the configuration according to its settings.
+
+The built in configuration would correspond to the following turtle.cfg::
+
+   width = 0.5
+   height = 0.75
+   leftright = None
+   topbottom = None
+   canvwidth = 400
+   canvheight = 300
+   mode = standard
+   colormode = 1.0
+   delay = 10
+   undobuffersize = 1000
+   shape = classic
+   pencolor = black
+   fillcolor = black
+   resizemode = noresize
+   visible = True
+   language = english
+   exampleturtle = turtle
+   examplescreen = screen
+   title = Python Turtle Graphics
+   using_IDLE = False
+
+Short explanation of selected entries:
+
+- The first four lines correspond to the arguments of the :meth:`Screen.setup`
+  method.
+- Line 5 and 6 correspond to the arguments of the method
+  :meth:`Screen.screensize`.
+- *shape* can be any of the built-in shapes, e.g: arrow, turtle, etc.  For more
+  info try ``help(shape)``.
+- If you want to use no fillcolor (i.e. make the turtle transparent), you have
+  to write ``fillcolor = ""`` (but all nonempty strings must not have quotes in
+  the cfg-file).
+- If you want to reflect the turtle its state, you have to use ``resizemode =
+  auto``.
+- If you set e.g. ``language = italian`` the docstringdict
+  :file:`turtle_docstringdict_italian.py` will be loaded at import time (if
+  present on the import path, e.g. in the same directory as :mod:`turtle`.
+- The entries *exampleturtle* and *examplescreen* define the names of these
+  objects as they occur in the docstrings.  The transformation of
+  method-docstrings to function-docstrings will delete these names from the
+  docstrings.
+- *using_IDLE*: Set this to ``True`` if you regularly work with IDLE and its -n
+  switch ("no subprocess").  This will prevent :func:`exitonclick` to enter the
+  mainloop.
+
+There can be a :file:`turtle.cfg` file in the directory where :mod:`turtle` is
+stored and an additional one in the current working directory.  The latter will
+override the settings of the first one.
+
+The :file:`Demo/turtle` directory contains a :file:`turtle.cfg` file.  You can
+study it as an example and see its effects when running the demos (preferably
+not from within the demo-viewer).
+
+
+Demo scripts
+============
+
+There is a set of demo scripts in the turtledemo directory located in the
+:file:`Demo/turtle` directory in the source distribution.
+
+It contains:
+
+- a set of 15 demo scripts demonstrating differet features of the new module
+  :mod:`turtle`
+- a demo viewer :file:`turtleDemo.py` which can be used to view the sourcecode
+  of the scripts and run them at the same time. 14 of the examples can be
+  accessed via the Examples menu; all of them can also be run standalone.
+- The example :file:`turtledemo_two_canvases.py` demonstrates the simultaneous
+  use of two canvases with the turtle module.  Therefore it only can be run
+  standalone.
+- There is a :file:`turtle.cfg` file in this directory, which also serves as an
+  example for how to write and use such files.
+
+The demoscripts are:
+
++----------------+------------------------------+-----------------------+
+| Name           | Description                  | Features              |
++----------------+------------------------------+-----------------------+
+| bytedesign     | complex classical            | :func:`tracer`, delay,|
+|                | turtlegraphics pattern       | :func:`update`        |
++----------------+------------------------------+-----------------------+
+| chaos          | graphs verhust dynamics,     | world coordinates     |
+|                | proves that you must not     |                       |
+|                | trust computers' computations|                       |
++----------------+------------------------------+-----------------------+
+| clock          | analog clock showing time    | turtles as clock's    |
+|                | of your computer             | hands, ontimer        |
++----------------+------------------------------+-----------------------+
+| colormixer     | experiment with r, g, b      | :func:`ondrag`        |
++----------------+------------------------------+-----------------------+
+| fractalcurves  | Hilbert & Koch curves        | recursion             |
++----------------+------------------------------+-----------------------+
+| lindenmayer    | ethnomathematics             | L-System              |
+|                | (indian kolams)              |                       |
++----------------+------------------------------+-----------------------+
+| minimal_hanoi  | Towers of Hanoi              | Rectangular Turtles   |
+|                |                              | as Hanoi discs        |
+|                |                              | (shape, shapesize)    |
++----------------+------------------------------+-----------------------+
+| paint          | super minimalistic           | :func:`onclick`       |
+|                | drawing program              |                       |
++----------------+------------------------------+-----------------------+
+| peace          | elementary                   | turtle: appearance    |
+|                |                              | and animation         |
++----------------+------------------------------+-----------------------+
+| penrose        | aperiodic tiling with        | :func:`stamp`         |
+|                | kites and darts              |                       |
++----------------+------------------------------+-----------------------+
+| planet_and_moon| simulation of                | compound shapes,      |
+|                | gravitational system         | :class:`Vec2D`        |
++----------------+------------------------------+-----------------------+
+| tree           | a (graphical) breadth        | :func:`clone`         |
+|                | first tree (using generators)|                       |
++----------------+------------------------------+-----------------------+
+| wikipedia      | a pattern from the wikipedia | :func:`clone`,        |
+|                | article on turtle graphics   | :func:`undo`          |
++----------------+------------------------------+-----------------------+
+| yingyang       | another elementary example   | :func:`circle`        |
++----------------+------------------------------+-----------------------+
+
+Have fun!
+
+
+Changes since Python 2.6
+========================
+
+- The methods :meth:`Turtle.tracer`, :meth:`Turtle.window_width` and 
+  :meth:`Turtle.window_height` have been eliminated. 
+  Methods with these names and functionality are now available only 
+  as methods of :class:`Screen`. The functions derived from these remain
+  available. (In fact already in Python 2.6 these methods were merely 
+  duplications of the corresponding 
+  :class:`TurtleScreen`/:class:`Screen`-methods.)
+
+- The method :meth:`Turtle.fill` has been eliminated. 
+  The behaviour of :meth:`begin_fill` and :meth:`end_fill` 
+  have changed slightly: now  every filling-process must be completed with an 
+  ``end_fill()`` call.
+  
+- A method :meth:`Turtle.filling` has been added. It returns a boolean
+  value: ``True`` if a filling process is under way, ``False`` otherwise.
+  This behaviour corresponds to a ``fill()`` call without arguments in
+  Python 2.6.
+