copysign() returns x with the same sign as y. IEEE Std 754
    requires copysign(x,NaN) = +x or -x.

                           Entry-Point Names

    Generic
    Function   Data Type   OpenVMS            Tru64 UNIX
    Name       Required

    copysign   S_FLOAT     math$copysign_s    copysignf
               T_FLOAT     math$copysign_t    copysign
               X_FLOAT     math$copysign_x    copysignl
               F_FLOAT     math$copysign_f
               G_FLOAT     math$copysign_g