Return a set of all Ent subclasses, recursively.

def subclasses(cls):
        """Return a set of all Ent subclasses, recursively."""
        seen = set()
        queue = set([cls])

        while queue:
            c = queue.pop()
            seen.add(c)

            sc = c.__subclasses__()
            for c in sc:
                if c not in seen:
                    queue.add(c)

        seen.remove(cls)
        return seen