acos() computes the principal value of the arc cosine of x in the
    interval [0,pi] radians for x in the interval [-1,1].

    acosd() computes the principal value of the arc cosine of x in
    the interval [0,180] degrees for x in the interval [-1,1].

                           Entry-Point Names

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

    acos       S_FLOAT     math$acos_s    acosf
               T_FLOAT     math$acos_t    acos
               X_FLOAT     math$acos_x    acosl
               F_FLOAT     math$acos_f
               G_FLOAT     math$acos_g

    acosd      S_FLOAT     math$acosd_s   acosdf
               T_FLOAT     math$acosd_t   acosd
               X_FLOAT     math$acosd_x   acosdl
               F_FLOAT     math$acosd_f
               G_FLOAT     math$acosd_g