apply IsDeduct.mp_ ((forall_ v P.not_).imp_ (fastReplaceFree v t P).not_)
P : Formula
v t : VarName
h1 : fastAdmitsAux v t ∅ P
⊢ IsDeduct ∅ ((fastReplaceFree v t P).imp_ (forall_ v P.not_).not_)
case a
P : Formula
v t : VarName
h1 : fastAdmitsAux v t ∅ P
⊢ IsDeduct ∅
    (((forall_ v P.not_).imp_ (fastReplaceFree v t P).not_).imp_ ((fastReplaceFree v t P).imp_ (forall_ v P.not_).not_))

case a
P : Formula
v t : VarName
h1 : fastAdmitsAux v t ∅ P
⊢ IsDeduct ∅ ((forall_ v P.not_).imp_ (fastReplaceFree v t P).not_)