Functor F: C → D preserves composition: F(g∘f) = F(g)∘F(f) | F(id_A) = id_{F(A)}
Select "Commuting Diagram" and click Check
A category consists of objects and morphisms (arrows) between them, with associative composition
and identity morphisms. A functor maps categories structure-preservingly.
A natural transformation η: F ⇒ G maps between functors, with a component for each object.