static S dropWords(S s, L<S> words) {
  ret dropWords_impl(s, asSet(toLowerCase(words)));
}

static S dropWords(S s, S words) {
  ret dropWords(s, splitWords(words));
}

static S dropWords(S s, S... words) {
  ret dropWords_impl(s, asSet(toLowerCase(words)));
}

static Set<S> dropWords_preprocess(S words) {
  ret asSet(splitWords(toLowerCase(words)));
}

static S dropWords(S s, Set<S> set) {
  ret dropWords_impl(s, asCISet(set));
}

static S dropWords_impl(S s, Set<S> set) {
  L<S> tok = nlTok(s);
  for (int i = 1; i < l(tok); )
    if (set.contains(tok.get(i).toLowerCase()))
      removeToken(tok, i);
    else
      i += 2;
  ret join(tok);
}