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

    OpenVMS usage:address
    type:         address
    access:       write only
    mechanism:    by value

    Previous contents of SF$A_HANDLER (longword 0) of the caller's
    stack frame. This is the address of the condition handler
    previously in effect. If no condition handler was in effect,
    zero is returned.