basics, equivalences

[RobertHarper]

I'm reviewing, as promised, Chapter 3, but in the process found a typo and a minor unclarity (imho) of the proofs of Lemmas 2.8.11 and 2.8.12, which I have now changed slightly to correct the missing equals sign and make the proof a bit clearer.

The proof of Lemma 3.1.2 seems a little murky to me. I'm going to write out my own version and compare before making any suggestions for any changes.