case inl h1 =>
  split_ifs
  case pos c1 | neg c1 =>
    first | apply IsSub.forall_not_free_in | apply IsSub.exists_not_free_in
    subst h1
    simp only [isFreeIn]
    simp
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 : v = x
⊢ IsSub (exists_ x phi) v u (if v = x then exists_ x phi else exists_ x (Rec.fastReplaceFree v u phi))
no goals