 Set. The objects are the sets, and the morphisms   are triples  ⟨x, y, f⟩,  where f : x → y.  That is, each triple ⟨ x, y, f⟩ is a morphism from x to y.  The identity morphism ida for a set a is ⟨a, a, f⟩ , where f is the identity function on a and the composition operation of morphisms is given by: 