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

 invo_handle

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

    An invocation handle that represents a target invocation context.