def get_parentmap_toposort(self, reverse=False):
"""
Returns an iterator in topological order over element, parent pairs.
"""
parent_map = self.get_parentmap()
digraph = graph.digraph(parent_map.items())
if reverse:
digraph = graph.reverse(digraph)
for element in toposort_flatten(digraph, sort=False):
yield element, parent_map.get(element, None)