def renderable(tiddler, environ=None):
    """
    Return ``True`` if the provided :py:class:`tiddler's
    <tiddlyweb.model.tiddler.Tiddler>` ``type`` is one that can be
    rendered to HTML by the :py:mod:`wikitext <tiddlyweb.wikitext>`
    rendering subsystem.
    """
    if not environ:
        environ = {}
    return (not tiddler.type or tiddler.type == 'None' or tiddler.type in
        environ.get('tiddlyweb.config', {}).get('wikitext.type_render_map', [])
        )
