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Exercise 1:

• Improve the last symplification function of lesson 5 by adding the simplification rule $$x - 0 = x$$

Exercise 2:

• Improve on exercise 1 by adding to the expr inductive type a constructor for multiplication and the simplification rules $$x * 0 = 0$$ and $$0 * x = 0$$

Exercise 3:

• Improve on exercise 2 by adding to the expr data type a constructor for One and the simplification rules $$x * 1 = x$$ and $$1 * x = x$$