sort.py

    # sort the stages
    out_degree_map = {v: d for v, d in g.out_degree() if d > 0}
    zero_out_degree = [v for v, d in g.out_degree() if d == 0]
    while zero_out_degree:
        yield zero_out_degree
        new_zero_out_degree = []
        for v in zero_out_degree:
            for child, _ in g.in_edges(v):
                out_degree_map[child] -= 1
                if not out_degree_map[child]:
                    new_zero_out_degree.append(child)
        zero_out_degree = new_zero_out_degree