Purpose:

The purpose of this problem set is to understand the mechanisms of abstractions in an object-oriented programming language, especially (abstract) classes, inheritance, and overriding.

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Drill:

HtDC: All problems in part iv. Ignore all problems that ask you to draw things.

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Required Problems:

Note: You must use the Module language level in DrScheme and you must use the language #lang scheme.

1. "Edit" your solution of problem 10.1/11.1. Then add a class for representing subtraction expressions, which have exactly two sub-expressions.

2. Turn your solution of 10.2/11.2 into a complete class-oriented solution, using the base class world% from the class-universe.ss teachpack. The teachpack exports four items: world<%>, world%, place-image, and empty-scene. The first two describe a base class for creating world programs. The latter two are from universe.ss.

   Then "edit" your solution of problem 10.2/11.2.