Perform a depth-first search on the given DiGraph, with a limit on maximum steps.

        :param networkx.DiGraph graph:  The graph to traverse.
        :param Any source:              The source to begin traversal.
        :param int max_steps:           Maximum steps of the traversal, or None if not limiting steps.
        :return: An iterator of edges.

def _dfs_edges(graph, source, max_steps=None):
        """
        Perform a depth-first search on the given DiGraph, with a limit on maximum steps.

        :param networkx.DiGraph graph:  The graph to traverse.
        :param Any source:              The source to begin traversal.
        :param int max_steps:           Maximum steps of the traversal, or None if not limiting steps.
        :return: An iterator of edges.
        """

        if max_steps is None:
            yield networkx.dfs_edges(graph, source)

        else:
            steps_map = defaultdict(int)
            traversed = { source }
            stack = [ source ]

            while stack:
                src = stack.pop()
                for dst in graph.successors(src):
                    if dst in traversed:
                        continue
                    traversed.add(dst)

                    dst_steps = max(steps_map[src] + 1, steps_map[dst])

                    if dst_steps > max_steps:
                        continue

                    yield src, dst

                    steps_map[dst] = dst_steps
                    stack.append(dst)