atanh() returns the hyperbolic arc tangent of x for x in the
    interval (-1,1). atanh() is the inverse function of tanh().
    atanh(tanh (x)) = x.

    atanh(x) is computed as 1/2 ln((1+x)/(1-x)).

                           Entry-Point Names

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

    atanh      S_FLOAT     math$atanh_s   atanhf
               T_FLOAT     math$atanh_t   atanh
               X_FLOAT     math$atanh_x   atanhl
               F_FLOAT     math$atanh_f
               G_FLOAT     math$atanh_g