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

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

    Status value. A value of 1 indicates success. When the initial
    context represents the bottom of the call chain, a value of 0 is
    returned.