construct proofs (in the same fashion that was demonstrated in the video lectures)
to show that the following sequents are valid.
1. (–q --> –p), p ⊢ q
2. (p --> (m --> r), m ⊢ p--> r
3. w--> (s--> (b--> r)) ⊢b--> (w--> (s--> r))
4. –t--> –r ⊢ r--> t

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