Prove $\vdash((\alpha\rightarrow\beta)\rightarrow\gamma)\rightarrow(\neg \gamma\rightarrow\alpha)$ in $P$.
In propositional calculus formal system $P$, three axioms could be used:
(A1) $(\alpha\rightarrow(\beta\rightarrow\alpha))$
(A2) $((\alpha\rightarrow(\beta\rightarrow\gamma))\rightarrow((\alpha\rightarrow\beta)\rightarrow(\alpha\rightarrow\gamma)))$
(A3) $(((\neg\alpha)\rightarrow(\neg\beta))\rightarrow(\beta\rightarrow\alpha))$
I've already proved it in N, and I understand the equivalence of N and P. But I wonder how to prove the proposition directly in P using A1~A3.