drem() returns the remainder r = x-n*y, where n = rint(x/y).
    Additionally, if |n-x/y|=1/2, then n is even. The remainder is
    computed exactly, and |r| is less than or equal to |y|/2. The
    drem() and remainder()  functions are aliases of each other.

                           Entry-Point Names

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

    drem       S_FLOAT     math$rem_s     dremf
               T_FLOAT     math$rem_t     drem
               X_FLOAT     math$rem_x     dreml
               F_FLOAT     math$rem_f
               G_FLOAT     math$rem_g