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+:mod:`__future__` --- Future statement definitions
+==================================================
+
+.. module:: __future__
+   :synopsis: Future statement definitions
+
+
+:mod:`__future__` is a real module, and serves three purposes:
+
+* To avoid confusing existing tools that analyze import statements and expect to
+  find the modules they're importing.
+
+* To ensure that future_statements run under releases prior to 2.1 at least
+  yield runtime exceptions (the import of :mod:`__future__` will fail, because
+  there was no module of that name prior to 2.1).
+
+* To document when incompatible changes were introduced, and when they will be
+  --- or were --- made mandatory.  This is a form of executable documentation, and
+  can be inspected programatically via importing :mod:`__future__` and examining
+  its contents.
+
+Each statement in :file:`__future__.py` is of the form::
+
+   FeatureName = "_Feature(" OptionalRelease "," MandatoryRelease ","
+                           CompilerFlag ")"
+
+
+where, normally, *OptionalRelease* is less than *MandatoryRelease*, and both are
+5-tuples of the same form as ``sys.version_info``::
+
+   (PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
+    PY_MINOR_VERSION, # the 1; an int
+    PY_MICRO_VERSION, # the 0; an int
+    PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string
+    PY_RELEASE_SERIAL # the 3; an int
+   )
+
+*OptionalRelease* records the first release in which the feature was accepted.
+
+In the case of a *MandatoryRelease* that has not yet occurred,
+*MandatoryRelease* predicts the release in which the feature will become part of
+the language.
+
+Else *MandatoryRelease* records when the feature became part of the language; in
+releases at or after that, modules no longer need a future statement to use the
+feature in question, but may continue to use such imports.
+
+*MandatoryRelease* may also be ``None``, meaning that a planned feature got
+dropped.
+
+Instances of class :class:`_Feature` have two corresponding methods,
+:meth:`getOptionalRelease` and :meth:`getMandatoryRelease`.
+
+*CompilerFlag* is the (bitfield) flag that should be passed in the fourth
+argument to the builtin function :func:`compile` to enable the feature in
+dynamically compiled code.  This flag is stored in the :attr:`compiler_flag`
+attribute on :class:`_Feature` instances.
+
+No feature description will ever be deleted from :mod:`__future__`.
+