The features of Turing described in the previous ten chapters are adequate for almost all programs. There are some situations, however, in which additional features are useful. We can divide additional features into two categories. The first category consists of features that are essential in the sense that we cannot write programs to perform certain tasks if we cannot use these features. The second category consists of features that are luxuries in the sense that they provide convenient solutions to problems that would be hard to solve using simpler features.
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