Added bastion and (dummy) rexec docs

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+\section{Standard Module \sectcode{Bastion}}
+\stmodindex{Bastion}
+\renewcommand{\indexsubitem}{(in module Bastion)}
+
+% I'm concerned that the word 'bastion' won't be understood by people
+% for whom English is a second language, making the module name
+% somewhat mysterious.  Thus, the brief definition... --amk
+
+According to the dictionary, a bastion is ``a fortified area or
+position'', or ``something that is considered a stronghold.''  It's a
+suitable name for this module, which provides a way to forbid access
+to certain attributes of an object.  It must always be used with the
+\code{rexec} module, in order to allow restricted-mode programs access
+to certain safe attributes of an object, while denying access to
+other, unsafe attributes.
+
+% I've punted on the issue of documenting keyword arguments for now.
+
+\begin{funcdesc}{Bastion}{object\optional{\, filter\, name\, class}}
+Protect the class instance \var{object}, returning a bastion for the
+object.  Any attempt to access one of the object's attributes will
+have to be approved by the \var{filter} function; if the access is
+denied an AttributeError exception will be raised.
+
+If present, \var{filter} must be a function that accepts a string
+containing an attribute name, and returns true if access to that
+attribute will be permitted; if \var{filter} returns false, the access
+is denied.  The default filter denies access to any function beginning
+with an underscore (\code{_}).  The bastion's string representation
+will be \code{<Bastion for \var{name}>} if a value for
+\var{name} is provided; otherwise, \code{repr(\var{object})} will be used.
+
+\var{class}, if present, would be a subclass of \code{BastionClass};
+see the code in \file{bastion.py} for the details.  Overriding the
+default \code{BastionClass} will rarely be required.  
+
+\end{funcdesc}