doctests.

        This returns a modified version of the input string.  It looks
        for doctests, which start with a ">>>" prompt, and may be
        continued with "..." prompts, as long as the "..." is indented
        the same as the ">>>".

        (Unfortunately we can't use the doctest module's parser,
        since, like most parsers, it is not geared towards preserving
        the original source.)
        Nr