def knot_insertion_alpha(u, knotvector, span, idx, leg):
    """ Computes :math:`\\alpha` coefficient for knot insertion algorithm.

    :param u: knot
    :type u: float
    :param knotvector: knot vector
    :type knotvector: tuple
    :param span: knot span
    :type span: int
    :param idx: index value (degree-dependent)
    :type idx: int
    :param leg: i-th leg of the control points polygon
    :type leg: int
    :return: coefficient value
    :rtype: float
    """
    return (u - knotvector[leg + idx]) / (knotvector[idx + span + 1] - knotvector[leg + idx])