def get_category(category_name, default=None, sort_key=None):
    """ Get a sequence of dictionaries in the form
        ``[{'introspectable':IIntrospectable, 'related':[sequence of related
        IIntrospectables]}, ...]`` where each introspectable is part of the
        category associated with ``category_name`` .

        If the category named ``category_name`` does not exist in the
        introspector the value passed as ``default`` will be returned.

        If ``sort_key`` is ``None``, the 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)."""
