apply IsDeduct.assume_
case h1.h2.a.h2.a
P Q : Formula
v : VarName
⊢ IsDeduct ({exists_ v (P.and_ Q)} ∪ {P.and_ Q}) (P.and_ Q)
case h1.h2.a.h2.a.a
P Q : Formula
v : VarName
⊢ P.and_ Q ∈ {exists_ v (P.and_ Q)} ∪ {P.and_ Q}