static S structure_addTokenMarkers(S s) {
  L<S> tok = javaTok(s);
  
  // find references
  
  new TreeSet<Int> refs;
  for (int i = 1; i < l(tok); i += 2) {
    S t = tok.get(i);
    if (t.startsWith("t") && isInteger(t.substring(1)))
      refs.add(parseInt(t.substring(1)));
  }
  
  if (empty(refs)) ret s;
  
  // add markers
  for (int i : refs) {
    int idx = i*2+1;
    tok.set(idx, "m" + i + " " + tok.get(idx));
  }
  
  ret join(tok);
}