Sometimes a Processing class needs to become a hierarchical structure that contains other Processing classes. Such container Processing classes are called Processing Composites. For implementing such structure, instead of deriving from the Processing base class, the ProcessingComposite class must be used.

The only difference from a regular Processing class is that child Processing objects must be configured in the ConcreteConfigure operation (usually through the use of an auxiliary ConfigureChildren operation) and they also need to be ``attached'' to their parent.