The Sequence and Alternation classes are subclasses of CollectionParser , as Figure 10.3 shows. These classes have a common way of adding and storing their subparsers. They differ primarily in how they match a set of assemblies.
Figure 10.3. The CollectionParser class. CollectionParser is the abstract superclass of the two fundamental types of collection parsers: Alternation and Sequence .
The main behavior that CollectionParser provides to its subclasses is the ability to add a parser to a collection.