∀ {G : Type u_1} {H : Type u_2} [inst : AddCommMonoid G] [inst_1 : AddCommMonoid H] {A : Set (G × G)} {s : Set G}
  {t : Set H} {f : G → H} {x₁ y₁ x₂ y₂ : G},
  IsAddFreimanIso 2 s t f →
    A ⊆ s ×ˢ s →
      x₁ ∈ s →
        y₁ ∈ s → x₂ ∈ s → y₂ ∈ s → IsCorner (Prod.map f f '' A) (f x₁) (f y₁) (f x₂) (f y₂) → IsCorner A x₁ y₁ x₂ y₂