sbool reTok_modify_check;

static LS lambdaMapLike reTok_modify(IF1<LS> f, L<S> tok, int i, int j) {
  ret reTok_modify(tok, i, j, f);
}

// f: func(L<S>) -> L<S>
static L<S> reTok_modify(L<S> tok, int i, int j, O f) {
  // extend i to an "N" token
  // and j to "C" (so j-1 is an "N" token)
  i = i & ~1;
  j = j | 1;
  
  L<S> t = javaTok(joinSubList(tok, i, j));
  if (f != null) {
    t = (L<S>) callF(f, t);
    if (reTok_modify_check)
      assertEquals("Improperly tokenized", javaTok(join(t)), t);
  }
  replaceListPart(tok, i, j, t);
  
  ret tok;
}

Began life as a copy of #1003367