static S structure_addTokenMarkers(S s) {
  ret join(structure_addTokenMarkers(javaTokForStructure(s)));
}
  
static L<S> structure_addTokenMarkers(L<S> tok) {
  // 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 tok;
  
  // add markers
  for (int i : refs) {
    int idx = i*2+1;
    if (idx >= l(tok)) continue; // broken structure
    S t = "";
    if (endsWithLetterOrDigit(tok.get(idx-1))) t = " ";
    tok.set(idx, t + "m" + i + " " + tok.get(idx));
  }
  
  ret tok;
}