@NotNull
  @Override
  public DfReferenceType dropMutability() {
    return myMutability == Mutability.UNKNOWN ? this :
           new DfGenericObjectType(myNotValues, myConstraint, myNullability, Mutability.UNKNOWN, mySpecialField, mySpecialFieldType,
                                   myLocal);
  }
@NotNull
  @Override
  public DfReferenceType dropMutability()