sS jsonEncode_breakAtLevels(int levels default 5, O o) {

  new StringBuilder buf;

  jsonEncode_breakAtLevels(levels, o, buf, 1);

  ret str(buf);

}

static void jsonEncode_breakAtLevels(int levels, O o, StringBuilder buf, int level) {

  if (o == null)

    buf.append("null");

  else if (o instanceof S)

    buf.append(quote((S) o));

  else if (o instanceof Number || o instanceof Bool)

    buf.append(o);

  else if (o instanceof Map) {

    Map map = cast o;

    buf.append("{");

    bool first = true;

    for (O key : keys(map)) {

      if (first) first = false; else buf.append(",");

      if (level == 1) buf.append("\n" + spaces(level*2));

      buf.append(quote((S) key));

      buf.append(":");

      jsonEncode_breakAtLevels(levels, map.get(key), buf, level+1);

    }

    if (level <= levels) buf.append("\n" + spaces((level-1)*2));

    buf.append("}");

  } else if (o instanceof Collection) {

    Collection l = cast o;

    buf.append("[");

    bool first = true;

    for (O element : l) {

      if (first) first = false; else buf.append(",");

      if (level <= levels) buf.append("\n" + spaces(level*2));

      jsonEncode_breakAtLevels(levels, element, buf, level+1);

    }

    if (level <= levels) buf.append("\n" + spaces((level-1)*2));

    buf.append("]");

  } else

    fail("Unknown object for JSON encoding: " + className(o));

}