@nx.utils.not_implemented_for('undirected')
def is_arborescence(G):
    """
    Returns True if `G` is an arborescence.

    An arborescence is a directed tree with maximum in-degree equal to 1.

    Parameters
    ----------
    G : graph
        The graph to test.

    Returns
    -------
    b : bool
        A boolean that is True if `G` is an arborescence.

    Notes
    -----
    In another convention, an arborescence is known as a *tree*.

    See Also
    --------
    is_tree

    """
    return is_tree(G) and 1 <= max(d for n, d in G.in_degree())
dataset/ETHPy150Open networkx/networkx/networkx/algorithms/tree/recognition.py/is_arborescence/swapped operands of `<=`