VMS Help RTL Routines, MTH$, MTH$xACOS, Returns  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    OpenVMS usage:floating_point
    type:         F_floating, D_floating, G_floating
    access:       write only
    mechanism:    by value

    Angle in radians. MTH$ACOS returns an F-floating number.
    MTH$DACOS returns a D-floating number. MTH$GACOS returns a G-
    floating number.