static ItIt<PairS> ai_findLeadingPhrases(S s, Collection<S> phrases) {
  final Set<S> set = asCISet(phrases);
  L<S> tok = javaTokNPunctuation(s);
  ret notNulls_iterator(
    mapIterator(allTokSplits_nOnLeft_iterator(tok),
      func(Pair<LS> p) -> PairS {
        set.contains(trimJoin(simpleSpaces(p.a))) ? pair(join(p.a), join(p.b)) : null
      }));
}