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Just a heads up, worksheet 5 has an error: (P ↔ Q) → (R ↔ S) → (P ∧ Q ↔ R ∧ S). That proposition is not actually true.

It’s probably supposed to be (P & R) <-> (Q & S)

Yup, transposition error.

Thanks so much! Fixed.

PS I cannot believe my undergraduate teaching material is on HN! I am a math lecturer and this is just my course notes for my UGs.

It's very fun. Thanks for putting it together.