Proof
fun (F G : Ens) (f : well_definedness F) (g : well_definedness G) =>
Build_is_a_function (COMP F G) (COMP_th1 F G)
(composition_well_definedness F G f g).

Theorem composition_th2 :
forall (F G : Ens) (f : is_a_function F) (g : is_a_function G),
is_a_function (COMP F G).