nextafter() returns the machine-representable number next to x in
    the direction y.

                           Entry-Point Names

    Generic                                       Compaq
    Function   Data Type                          Tru64 UNIX
    Name       Required    OpenVMS Alpha          Alpha

    nextafter  S_FLOAT     math$nextafter_s       nextafterf
               T_FLOAT     math$nextafter_t       nextafter
               X_FLOAT     math$nextafter_x       nextafterl
               F_FLOAT     math$nextafter_f
               G_FLOAT     math$nextafter_g