r finding the zero of a
           nonlinear function without using derivatives".
           Advances in Engineering Software, 28(3), 145-149.
           https://doi.org/10.1016/s0965-9978(96)00051-8

    Examples
    --------
    First evaluate `gdtr`.

    >>> from scipy.special import gdtr, gdtrib
    >>> p = gdtr(1.2, 3.4, 5.6)
    >>> print(p)
    0.94378087442

    Verify the inverse.

    >>> gdtrib(1.2, p, 5.6)
    3.3999999999999995
    Ú