apply deduction_theorem
case h1
P Q : Formula
v : VarName
⊢ IsDeduct (∅ ∪ {forall_ v (P.iff_ Q)}) ((exists_ v P).imp_ (exists_ v Q))
case h1.h1
P Q : Formula
v : VarName
⊢ IsDeduct (∅ ∪ {forall_ v (P.iff_ Q)} ∪ {exists_ v P}) (exists_ v Q)