def topologically_sorted_indexes(self):
        """Iterate in topological order, based on networkx's topological_sort() function.
        """
        try:
            return self._topologcally_sorted_indexes_cache
        except AttributeError:
            seen = set()
            order = []
            explored = set()

            for i in self.edges:
                if i in explored:
                    continue
                fringe = [i]
                while fringe:
                    w = fringe[-1]  # depth first search
                    if w in explored:  # already looked down this branch
                        fringe.pop()
                        continue
                    seen.add(w)  # mark as seen
                    # Check successors for cycles and for new nodes
                    new_nodes = []
                    for n in self.outputs_of(w):
                        if n not in explored:
                            if n in seen:  # CYCLE !!
                                raise RuntimeError("Graph contains a cycle.")
                            new_nodes.append(n)
                    if new_nodes:  # Add new_nodes to fringe
                        fringe.extend(new_nodes)
                    else:  # No new nodes so w is fully explored
                        explored.add(w)
                        order.append(w)
                        fringe.pop()  # done considering this node
            self._topologcally_sorted_indexes_cache = tuple(filter(lambda i: type(i) is int, reversed(order)))
            return self._topologcally_sorted_indexes_cache
Iterate in topological order, based on networkx's topological_sort() function.