Natural Deduction Proof $\neg(P \to Q) \vdash Q \to P$ I am trying to answer Question 3(e) in Exercise 1.2 of Huth and Ryan's Logic in Computer Science book for revision and I am stuck on it. The question asks you to prove the validity of the following sequent:
$\neg(P \to Q) \vdash  Q \to P$
Any help would be appreciated.

1) $\lnot (P \to Q)$ --- premise
2) $Q$ --- assumed [a]
3) $P \to Q$ --- from 2) by $\to$-intro
4) $\bot$ --- contradiction from 1) and 3)
5) $P$ --- from 4) by $\bot$-eim

6) $Q \to P$ --- from 2) and 5) by $\to$-intro, discharging [a].

