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