case diff_ h1_P h1_P' h1_1 h1_2 =>
  subst h1_1
  subst h1_2
  apply Forall_spec_id
U V P_U P_V : Formula
l : List VarName
h1_P h1_P' : Formula
h1_1 : h1_P = U
h1_2 : h1_P' = V
h2 : ∀ (v : VarName), (isFreeIn v U ∨ isFreeIn v V) ∧ isBoundIn v h1_P → v ∈ l
⊢ IsProof ((Forall_ l (U.iff_ V)).imp_ (h1_P.iff_ h1_P'))
no goals