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

    OpenVMS usage:invo_handle
    type:         longword (unsigned)
    access:       write only
    mechanism:    by value

    An invocation handle for the invocation context that is previous
    to that which was specified as the target.