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