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