∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} [inst : EquivBEq α] [inst_1 : LawfulHashable α]
  (m₁ m₁_1 : Std.ExtHashSet α), m₁ = m₁_1 → ∀ (m₂ m₂_1 : Std.ExtHashSet α), m₂ = m₂_1 → m₁.diff m₂ = m₁_1.diff m₂_1