docs: private static String mkString(TreeSet<Integer> values) {
          StringBuilder builder = new StringBuilder();
          builder.append("\033["); // escape.
          boolean first = true;
          for (int i : values) {
              if (first) {
                  first = false;
              } else {
                  builder.append(';');
              }
              builder.append(String.format("%d", i));
          }
          builder.append("m");
          return builder.toString();
      }