static LS ai_dropTokenRefNumbers(LS tok) {
  tok = tok_combineSquareBrackets_keep(tok);
  new LS out;
  for i over tok: {
    S t = get(tok, i+1);
    if (even(i) && isSquareBracketed(t)) ++i;
    else
      out.add(tok.get(i));
  }
  ret out;
}