The only way to realize these objects is by realizing objects they contain. This notation is itself a valid realization. The second way depicted on the right uses color to encode the different types of objects.
Proximity still plays an important role in the latter case.
Color encoding can be used to denote the type of objects involved. For example a combination of picture 5 and the second part of picture 4. Note that the other conventions for realizing composite objects lend themselves to automatic placement of secondary objects, which is not generally the case when using the attachment method. By using animation relations between objects and type of objects can be conveyed. Figure 5.3 shows two examples.