VMS Help RTL Routines, MTH$, MTH$xATAN2, 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$ATAN2 returns an F-floating number.
    MTH$DATAN2 returns a D-floating number. MTH$GATAN2 returns a
    G-floating number.