anchor.diff:  procedure TNullableString.SetValue(AValue: String);
   begin
     InternalSetValue(AValue);
  -  FTagChanged:= false;
  +  SetTagChanged(false);
   end;
   
   procedure TNullableString.InternalSetValue(AValue: String);
   begin
     FValue:= aValue;
  -  FIsNull:= False;
  +  SetIsNull(False);
   end;
   
   constructor TNullableString.Create();
  
positive.code:     else
        raise Exception.Create('TNullableBoolean: cannot assign value from field of type ' + Fieldtypenames[aSourceField.DataType]);
    end;
    FTagChanged:= false;
  end;
  
  procedure TNullableBoolean.Assign(const aValue: Variant);
  
negative.code: begin
    tmp := aValue;
    Self.Assign(tmp);
    SetTagChanged(false);
  end;
  
  function TNullableColor.CheckIfDifferentAndAssign(const aValue: Variant): boolean;
  