1. Peer review

Taking turns, each member of your group should seek peer review about their solutions to hw03. Although scrutiny of any of hw03 is a good idea, make sure to discuss at least the `simpleSum`/`simpleTerm`/`simpleFactor` functions.

Compare different styles and approaches to the problems. How did the others figure out the answers? Do any of you have lingering questions?

During this process, remember that everyone in this class has a different level of experience and be supportive of one anothers’ challenges. This is not a time to show off!

I expect this will take at least 20 minutes to get through everyone’s work.

2. Prove that `Succ Zero` is both a left-identity (e.g., `mult (Succ Zero) n = n`) and a right-identity (e.g., `mult n (Succ Zero) = n`) of `mult`. You may assume a lemma that says that, for all `n`, `plus n Zero = n`. Work with your partner(s) on this, and show me the final result. This is your chance to get feedback on a proof before next week’s exam. (You can structure your proof as two separate proofs; one for left-identity and one for right-identity.)

3. Write a `Monoid` instance for your `Tree` datatype from hw02. Challenge problem: Prove the monoid laws of your instance:
1. `m <> empty == m`
2. `empty <> m == m`
3. `(m1 <> m2) <> m3 == m1 <> (m2 <> m3)`
4. Begin hw04. Ask questions. The code for hw04 is not very intricate, but the setup is rather involved. Do take a close look at this before Sunday!