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

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