static Pair<L<S>> ai_dropCommonSuffixListCI_ifNonTrivial(Pair<L<S>> p) {
  int n = l(commonSuffixListCI(p));
  if (n == 0 || n == l(p.a) || n == l(p.b)) null;
  ret pair(takeFirst(n, p.a), takeFirst(n, p.b));
}