End com_arrow_def.
(* composition of two arrows in comma *)
Section comp_com_def.
Variables (axf bxg cxh : Com_ob) (f : Com_arrow axf bxg)
(g : Com_arrow bxg cxh).
Definition Comp_com_mor := Mor_com_arrow f o Mor_com_arrow g.

(* Specifications: *)
Lemma Comp_com_law : Com_law Comp_com_mor.
Canonical Structure Comp_com_arrow := Build_Com_arrow Comp_com_law.