sclass PhraseCache_rarestWord {
  // values: pairs of (rarest word index, phrase tokens)
  MultiMap<S, Pair<Int, LS>> byRarestWord = ciMultiMap();
  F1<S, LS> tokenizer = f<S, LS> javaTokNPunctuation;
  
  // doesn't check if phrase is there already
  void add(S phrase, S rarestWord) {
    LS tok = callF(tokenizer, phrase);
    if (l(tok) == 1) ret;
    int ofs = min(1, indexOfIC(tok, rarestWord));
    byRarestWord.put(tok.get(ofs), pair(ofs, tok));
  }
}