isnan() returns 1 (true) if x is NaN (the IEEE floating-point
    reserved Not-a-Number value) and 0 (false) otherwise.

                           Entry-Point Names

    Generic
    Function   Data Type   OpenVMS        Tru64 UNIX
    Name       Required

    isnan      S_FLOAT     math$isnan_s   isnanf
               T_FLOAT     math$isnan_t   isnan
               X_FLOAT     math$isnan_x   isnanl
               F_FLOAT     math$isnan_f
               G_FLOAT     math$isnan_g