static S replaceWords(S s, S... replacements) {
  Map<S, S> map = litmap(toLowerCase(replacements));
  L<S> tok = nlTok(s);
  for (int i = 1; i < l(tok); i += 2) {
    S x = map.get(tok.get(i).toLowerCase());
    if (x != null)
      tok.set(i, x);
  }
  ret join(tok);
}