Add an experimental mechanism to support extending the pprint formatting.

Partly responds to SF bug #505152.

Doc/lib/libpprint.tex

@@ -173,3 +173,26 @@ this returns false.
 \begin{methoddesc}{isrecursive}{object}
 Determine if the object requires a recursive representation.
 \end{methoddesc}
+
+This method is provided as a hook to allow subclasses to modify the
+way objects are converted to strings.  The default implementation uses
+the internals of the \function{saferepr()} implementation.
+
+\begin{methoddesc}{format}{object, context, maxlevels, level}
+Returns three values: the formatted version of \var{object} as a
+string, a flag indicating whether the result is readable, and a flag
+indicating whether recursion was detected.  The first argument is the
+object to be presented.  The second is a dictionary which contains the
+\function{id()} of objects that are part of the current presentation
+context (direct and indirect containers for \var{object} that are
+affecting the presentation) as the keys; if an object needs to be
+presented which is already represented in \var{context}, the third
+return value should be true.  Recursive calls to the \method{format()}
+method should add additionaly entries for containers to this
+dictionary.  The fourth argument, \var{maxlevels}, gives the requested
+limit to recursion; this will be \code{0} if there is no requested
+limit.  This argument should be passed unmodified to recursive calls.
+The fourth argument, \var{level} gives the current level; recursive
+calls should be passed a value less than that of the current call.
+\versionadded{2.3}
+\end{methoddesc}