@Override
    public String toString() {
      String removed = StreamEx.of(myRemovedRelations).map(Object::toString).append(myOldType.toString())
        .without("").joining(", ");
      String added = StreamEx.of(myAddedRelations).map(Object::toString).append(myNewType.toString())
        .without("").joining(", ");
      return (removed.isEmpty() ? "" : "-{" + removed + "} ") + (added.isEmpty() ? "" : "+{" + added + "}");
    }
@Override
    public String toString()