@Nullable
    static Change create(Set<Relation> removedRelations, Set<Relation> addedRelations, DfType oldType, DfType newType) {
      if (removedRelations.isEmpty() && addedRelations.isEmpty() && oldType == DfType.BOTTOM && newType == DfType.BOTTOM) {
        return null;
      }
      return new Change(removedRelations, addedRelations, oldType, newType);
    }
@Nullable
    static Change create(Set<Relation> removedRelations, Set<Relation> addedRelations, DfType oldType, DfType newType)