'A `set` of all the frame classes present in this `TransformGraph`.'

@property
def frame_set(self):

    if (self._cached_frame_set is None):
        self._cached_frame_set = frm_set = set()
        for a in self._graph:
            frm_set.add(a)
            for b in self._graph[a]:
                frm_set.add(b)
    return self._cached_frame_set.copy()
