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