VMS Help RTL Routines, MTH$, MTH$xATANH, 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

    The hyperbolic arc tangent of hyperbolic-tangent. MTH$ATANH
    returns an F-floating number. MTH$DATANH returns a D-floating
    number. MTH$GATANH returns a G-floating number.