VMS Help CC, Messages, UNDEFVARMOD  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Message        <Context> the expression "<expr>" modifies the
                variable "<var>" more than once without an
                intervening sequence point.  This behavior is
                undefined.

 Description    The compiler has detected a case where the same
                variable has been modified more than once in an
                expression without a sequence point between the
                modifications.  Because what modification will occur
                last is not defined, this expression might produce
                different results on different platforms.

 User Action    Rewrite the expression so that each variable is
                modified only once.