sS structure_convertTokenMarkersToExplicit(S s) {

  ret join(structure_convertTokenMarkersToExplicit(javaTokForStructure(s)));

}

static LS structure_convertTokenMarkersToExplicit(LS tok) {

  // find references & convert to explicit

  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)));

      tok.set(i, "r" + substring(t, 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;

}