VMS Help RTL Routines, MTH$, MTH$xATAN, 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. The angle returned will have a value in the
    range:

    MTH$ATAN returns an F-floating number. MTH$DATAN returns a D-
    floating number. MTH$GATAN returns a G-floating number.