∀ (α : Type u_1) (β : Type u_2) [inst : AddMonoid α] [inst_1 : PartialOrder α] [inst_2 : AddMonoid β]
  [inst_3 : Preorder β], (OrderAddMonoidHom.fstₗ α β).comp (OrderAddMonoidHom.inlₗ α β) = OrderAddMonoidHom.id α