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Abstract: Programming languages serve a dual purpose: to communicate programs to computers, and to communicate programs to humans. Indeed, it is this dual purpose that makes programming language design a constrained and challenging problem. Inheritance is an essential aspect of that second purpose: it is a tool to improve communication. Humans understand new concepts most readily by first looking at a number of concrete examples, and later abstracting over those examples. The essence of inheritance is that it mirrors this process: it provides a formal mechanism for moving from the concrete to the abstract.