case pos c1 =>
  first | apply IsSub.forall_not_free_in | apply IsSub.exists_not_free_in
  simp only [isFreeIn]
  tauto
v u x : VarName
phi : Formula
phi_ih : ∀ (binders : Finset VarName), Rec.fastAdmitsAux v u binders phi → IsSub phi v u (Rec.fastReplaceFree v u phi)
binders : Finset VarName
h1 : Rec.fastAdmitsAux v u (binders ∪ {x}) phi
c1 : v = x
⊢ IsSub (exists_ x phi) v u (exists_ x phi)
no goals