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

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