code: private UnicodeSet add_unchecked(int start, int end) {
          if (start < MIN_VALUE || start > MAX_VALUE) {
              throw new IllegalArgumentException("Invalid code point U+" + Utility.hex(start, 6));
          }
          if (end < MIN_VALUE || end > MAX_VALUE) {
              throw new IllegalArgumentException("Invalid code point U+" + Utility.hex(end, 6));
          }
          if (start < end) {
              add(range(start, end), 2, 0);
          } else if (start == end) {
              add(start);
          }
          return this;
      }
docs: for internal use, after checkFrozen has been called