##
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 $$1 * x = x$$