VMS Help CMS, INSERT, GENERATION, Qualifiers, /IF_ABSENT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Directs CMS to insert the element generation into the class only
    if a generation of that element is not already in the class. If a
    generation of the element is already in the class, CMS takes no
    action and does not return an error.