sclass WordIndex<A> {
  S regexp = "\\w+";
  MultiMap<S, A> index = ciMultiMap();
  
  void add(A a, S text) {
    Set<S> words = extractWords(text);
    for (S word : words) index.add(word, a);
  }
  
  Set<S> extractWords(S text) {
    ret asCISet(regexpExtractAll(regexp, text));
  }
  
  L<A> get(S word) {
    ret index.get(word);
  }
  
  void remove(A a, S text) {
    Set<S> words = extractWords(text);
    for (S word : words) index.remove(word, a);
  }
}