def intersection_abscissae(l1, l2):
    """ Return the intersection abscissae between l1 and l2 """
    if l1.is_parallel(l2):
        return None, None
    else:
        denominator = 1.0 / l1.v.cross(l2.v)
        delta = l2.p - l1.p
        s1 = delta.cross(l2.v) * denominator
        s2 = delta.cross(l1.v) * -denominator
        return s1, s2
