Returns a function that adds an edge to the `graph` checking only the out
    node.

    :param graph:
        A directed graph.
    :type graph: networkx.classes.digraph.DiGraph

    :return:
        A function that adds an edge to the `graph`.
    :rtype: callable

def add_edge_fun(graph):
    """
    Returns a function that adds an edge to the `graph` checking only the out
    node.

    :param graph:
        A directed graph.
    :type graph: networkx.classes.digraph.DiGraph

    :return:
        A function that adds an edge to the `graph`.
    :rtype: callable
    """

    # Namespace shortcut for speed.
    succ, pred, node = graph._succ, graph._pred, graph._node

    def add_edge(u, v, **attr):
        if v not in succ:  # Add nodes.
            succ[v], pred[v], node[v] = {}, {}, {}

        succ[u][v] = pred[v][u] = attr  # Add the edge.

    return add_edge