def categorized(sort_key=None):
    """ Get a sequence of tuples in the form ``[(category_name,
        [{'introspectable':IIntrospectable, 'related':[sequence of related
        IIntrospectables]}, ...])]`` representing all known
        introspectables.  If ``sort_key`` is ``None``, each introspectables
        sequence will be returned in the order the introspectables were added
        to the introspector.  Otherwise, sort_key should be a function that
        accepts an IIntrospectable and returns a value from it (ala the
        ``key`` function of Python's ``sorted`` callable)."""
