sS indentStructureString(int levels default 100, S s) {
  if (s == null) null;
  LS tok = javaTokForStructure(s);
  new StringBuilder buf;
  int indent = 0;
  levels = clampToInt(levels*2L);
  for i over tok: {
    S t = tok.get(i);
    if (isOpeningBracket(t)) {
      int j = i+5;
      if (containsClosingBracket(subList(tok, i+2, i+5))) {
        buf.append(joinSubList(tok, i, j));
        i = j-1;
      } else {
        indent += 2;
        buf.append(t);
        if (indent <= levels)
          buf.append("\n").append(spaces(indent));
      }
    } else if (isClosingBracket(t)) {
      indent -= 2;
      if (indent < levels)
        buf.append("\n").append(spaces(indent));
      buf.append(t);
    } else if (indent <= levels && eq(t, ",")) {
      buf.append(t).append("\n").append(spaces(indent));
      i++;
    } else
      buf.append(t);
  }
  ret str(buf);
}