simp only [def_exists_]
case h1.h1.h3
P Q : Formula
v : VarName
⊢ ∀ H ∈ {exists_ v P, forall_ v (P.iff_ Q)}, ¬isFreeIn v H
case h1.h1.h3
P Q : Formula
v : VarName
⊢ ∀ H ∈ {(forall_ v P.not_).not_, forall_ v (P.iff_ Q)}, ¬isFreeIn v H