anchor.diff:  
   { TAbstractNullable }
   
  +function TAbstractNullable.GetNotNull: Boolean;
  +begin
  +  Result := not FIsNull;
  +end;
  +
   constructor TAbstractNullable.Create;
   begin
     FIsNull := true;
  
positive.code:     property Value : Double read GetValue write SetValue;
    end;
  
  
  implementation
  
  { TNullableString }
  
  
negative.code:   TAbstractNullable = class abstract
    protected
      FIsNull : Boolean;
      function GetNotNull : Boolean;
    public
      constructor Create(); virtual;
      property IsNull : Boolean read FIsNull write FIsNull;
      property NotNull : Boolean read GetNotNull;
    end;
  
    { TNullableString }
  