def __edge_list(self):
    """
    Returns a tuple of two arrays (sources, targets) defining all the edges of the graph.

    :Example:

    >>> t = Tree((5, 5, 6, 6, 6, 7, 7, 7))
    >>> t.edge_list()
    (array([0, 1, 2, 3, 4, 5, 6]), array([5, 5, 6, 6, 6, 7, 7]))


    :return: pair of two 1d arrays
    """
    return self.sources(), self.targets()