fact: Definition RoofOb {x y : ob C} (R : Roof x y) : ob C := pr1 (pr1 R).
imports: UniMath.Foundations.PartD UniMath.Foundations.Propositions UniMath.Foundations.Sets UniMath.CategoryTheory.Core.Categories UniMath.CategoryTheory.Core.Isos UniMath.CategoryTheory.Core.Functors