static WordTokRule wordTokReplacerRuleOnXY(S sentence, S replaced) {
  ret wordTokReplacerRule(sentence, "X Y", replaced);
}

static WordTokRule wordTokReplacerRuleOnXY(S s) {
  PairS p = splitAtDoubleArrow_pair(s);
  if (p == null) { print("No arrow: " + s); null; }
  ret wordTokReplacerRuleOnXY(p.a, p.b);
}