// transformer can return null or input on no change

// will not call transformer on individual words

// returns null if no change

static WithTrail<S> lambdaMapLike gazelle_deepTransform_withTrail(IF1<S, WithTrail<S>> transformer, S input, O... _) {

  if (transformer == null) null;

  // call transformer on whole input, return if change

  WithTrail<S> transformed = transformer.get(input);

  if (transformed != null) ret transformed;

  // transformer didn't do anything, tokenize & go through parts

  O tokenizer = optPar tokenizer(_, f javaTokWithBrackets);

  LS tok = callTokenizer(tokenizer, input);

  new L trails;

  bool changes;

  for ping (int i = 1; i < l(tok); i += 2) {

    S t = tok.get(i);

    if (tok_isRoundOrCurlyBracketed(t)) {

      S unBracketed = tok_deRoundOrCurlyBracket(t);

      transformed = gazelle_deepTransform_withTrail(transformer, unBracketed, _);

      if (transformed != null) {

        if (!changes) tok = cloneList(tok); // in case a caching tokenizer was used

        // enclose in brackets & replace

        tok.set(i, startsWith(t, "(") ? roundOpt(transformed!) : curlyOpt(transformed!));

        trails.add(transformed.trail);

        set changes;

      }

    }

  }

  ret changes ? WithTrail(litmap(op := "gazelle_deepTransform_withTrail", +input, +trails), join(tok)) : null;

}