The most common motivation for extending the parser toolkit in sjm.parse is to provide a new Terminal subclass that recognizes a subset of tokens of an existing type, or that recognizes a new token type that you create. In addition, you may want to add behavior to the Parser hierarchy that provides a feature other than matching. The Track class is an example of an extension to the parsers in sjm.parse that does not increase the ability to match but adds value when the match fails.
In your own projects, you should feel free to extend, override, and replace the classes in any of the sjm packages. The same is true of the java packages and object-oriented programming in general. In the object-oriented era, the practice of programming has shifted from creating new programs from scratch to creating new programs by extending an existing set of software.
