fact: Definition opp_retract {C : category} {x y x' y' : C} {f : x --> y} {f' : x' --> y'} (r : retract f f') : retract (C:=op_cat C) (opp_mor f) (opp_mor f').
imports: UniMath.MoreFoundations.All UniMath.CategoryTheory.Core.Prelude UniMath.CategoryTheory.opp_precat