In the coming chapters, we'll make extensive use of the abstract base classes discussed in this chapter. In Chapter 6, Using Callables and Contexts, we'll look at the relatively simple features of callables and contexts. In Chapter 7, Creating Containers and Collections, we'll look at the available containers and collections. We'll also look at how to build a unique, new kind of container in this chapter. Lastly, in Chapter 8, Creating Numbers, we'll look at the various numeric types and how we can create our own kind of number.