anchor.diff:        expect(input, source);
         expect(output, target);
       });
  -    test('delegate', () {
  +    test('single reference', () {
         var source = lowercase();
         var input = source.setable();
         var target = uppercase();
  
positive.code:       expect(input.children.first, input.children.last);
        expect(output.equals(target & target), isTrue);
        expect(output.children.first, output.children.last);
      });
    });
    group('optimize', () {
  
negative.code:     });
    });
    group('transform', () {
      test('identity copy', () {
        var input = lowercase().setable();
        var output = transformParser(input, (parser) => parser);
        expect(input, isNot(output));
        expect(input.equals(output), isTrue);
        expect(input.children.single, isNot(output.children.single));
      });
      test('single parser', () {
        var source = lowercase();
        var input = source;
        var target = uppercase();
  