anchor.diff:  
       procedure Add (const aValue : integer); overload;
       procedure Add (const aValue : TNullableInteger); overload;
  +
  +    procedure Subtract (const aValue : integer); overload;
  +    procedure Subtract (const aValue : TNullableInteger); overload;
   
       class function StringToVariant(const aValue : String) : Variant;
       class function VariantToString(const aValue : Variant) : String;
  
positive.code: begin
    if aValue.NotNull then
      Self.Add(aValue.Value);
  end;
  
  class function TNullableDouble.StringToVariant(const aValue: String): Variant;
  
negative.code:     procedure Add(const aValue : double); overload;
      procedure Add(const aValue : TNullableDouble); overload;
  
      procedure Subtract(const aValue : double); overload;
      procedure Subtract(const aValue : TNullableDouble); overload;
  
  
      class function StringToVariant(const aValue: String): Variant;
      class function VariantToString(const aValue: Variant; const aDisplayFormat : string): String;
  
  