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

    Directs CMS to insert subgroup-expression into group-expression
    only if group-expression does not already contain it. If subgroup-
    expression already belongs to group-expression, CMS takes no
    action and does not return an error.