anchor.diff:      constructor Create(); override; overload;
       constructor Create(aValue: String); overload;
   
  -    procedure Assign(aSource : TNullableString); overload;
  -    procedure Assign(aSourceField : TField); override; overload;
  +    procedure Assign(const aSource : TNullableString); overload;
  +    procedure Assign(const aSourceField : TField); override; overload;
       procedure Assign(const aValue : String); override; overload;
       procedure Assign(const aValue : Variant); override; overload;
       procedure Assign(const  aValue : String; const aAllowBlankValue : boolean); overload;
  
positive.code:   FTagChanged:= false;
  end;
  
  procedure TNullableColor.Assign(aSourceField: TField);
  begin
    if aSourceField.IsNull then
      Self.IsNull:= true
  
negative.code:   Value:= aValue;
  end;
  
  procedure TNullableDoubleRecord.Assign(const aSource: TNullableDoubleRecord);
  begin
    IsNull:= aSource.IsNull;
    Value:= aSource.Value;
  end;
  
  procedure TNullableDoubleRecord.Assign(const aSourceField: TField);
  begin
    if aSourceField.IsNull then
      CreateAsNull
  