public String toString() {
      if (areEqual()) {
        return "equal";
      }

      StringBuilder result = new StringBuilder("not equal");
      if (!onlyOnLeft.isEmpty()) {
        result.append(": only on left=").append(onlyOnLeft);
      }
      if (!onlyOnRight.isEmpty()) {
        result.append(": only on right=").append(onlyOnRight);
      }
      if (!differences.isEmpty()) {