def _ggm_qcondwait_whitt_ds3(cs2):
    """
    Return the approximate E(V^3)/(EV)^2 where V is a service time; based on either a hyperexponential
    or Erlang distribution. Used in approximation of conditional wait time CDF (conditional on W>0).

    Whitt refers to conditional wait as D in his paper:

    See Whitt, Ward. "Approximations for the GI/G/m queue"
    Production and Operations Management 2, 2 (Spring 1993): 114-161.

    This is Equation 4.3 on p146. Note that there is a typo in the original paper in which the first term
    for Case 1 is shown as cubed, whereas it should be squared. This can be confirmed by seeing Eq 51 in
    Whitt's paper on the QNA (Bell Systems Technical Journal, Nov 1983).

    Parameters
    ----------
    cs2 : float
        squared coefficient of variation for service time distribution

    Returns
    -------
    float
        mean wait time in queue

    """

    if cs2 >= 1:
        ds3 = 3.0 * cs2 * (1.0 + cs2)
    else:
        ds3 = (2 * cs2 + 1.0) * (cs2 + 1.0)


    return ds3