a⇒b is a function object⟺∀z,∃!h:z→(a⇒b)∋g=eval∘(h×id)
- This relies on that a morphism between two products (h×id:(z×a)→((a⇒b)×a)) can be constructed as a product of two morphisms (h and id), so that we know that it is a correct way to obtain (a⇒b)×a from z×a using h:z→(a⇒b)