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