-Structure Retraction `{C:Category} `{D:Category} :=
-{ retraction_section_fobj : C -> D
-; retraction_section : Functor C D retraction_section_fobj
-; retraction_retraction_fobj : D -> C
-; retraction_retraction : Functor D C retraction_retraction_fobj
-; retraction_composes : retraction_section >>>> retraction_retraction ≃ functor_id _
-}.
-Implicit Arguments Retraction [ [Ob] [Hom] [Ob0] [Hom0] ].
-Coercion retraction_section : Retraction >-> Functor.