fact: Definition central `(f : x ~> y) : Type := ∀ x' y' (f' : x' ~> y'), (fmap[inj_left y'] f ∘ fmap[inj_right x] f' << x ⊗ x' ~~> y ⊗ y' >> fmap[inj_right y] f' ∘ fmap[inj_left x'] f) ∧ (fmap[inj_left y] f' ∘ fmap[inj_right x'] f << x' ⊗ x ~~> y' ⊗ y >> fmap[inj_right y'] f ∘ fmap[inj_left x] f').
imports: Category.Lib Category.Theory.Category Category.Theory.Functor