apply proof_imp_deduct
case a
P Q : Formula
Γ : Set Formula
h1 : IsProof P
h2 : IsDeduct Γ (P.imp_ Q)
⊢ IsDeduct Γ P
case a.h1
P Q : Formula
Γ : Set Formula
h1 : IsProof P
h2 : IsDeduct Γ (P.imp_ Q)
⊢ IsProof P