{A B} (f : A -> B) (C : Cyl f -> Type) (g : forall a, C (cyl a)) (ext : ExtensionAlong cyl C g) : ExtensionAlong cyl C g. Proof. srefine (Cyl_ind C g (ext.1 o cyr) _ ; _); intros a. + refine ((ext.2 a)^ @Dl _)%dpath. apply apD. + reflexivity. Defined.
Require Import HoTT.Basics HoTT.Types. Require Import Equiv.PathSplit Homotopy.IdentitySystems. Require Import Cubical.DPath Cubical.DPathSquare. Require Import Colimits.Coeq Colimits.MappingCylinder.