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