The distance between two skip-connections.

def skip_connection_distance(a, b):
    """The distance between two skip-connections."""
    if a[2] != b[2]:
        return 1.0
    len_a = abs(a[1] - a[0])
    len_b = abs(b[1] - b[0])
    return (abs(a[0] - b[0]) + abs(len_a - len_b)) / (max(a[0], b[0]) + max(len_a, len_b))