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

static WordTokRule wordTokReplacerRuleOnX(S s) {
  PairS p = splitAtDoubleArrow_pair(s);
  ret wordTokReplacerRuleOnX(p.a, p.b);
}