fact: Definition path_extension `{Funext} {A B : Type} {f : A -> B} {P : B -> Type} {d : forall x:A, P (f x)} (ext ext' : ExtensionAlong f P d) : (ExtensionAlong f (fun y => pr1 ext y = pr1 ext' y) (fun x => pr2 ext x @ (pr2 ext' x)^)) -> ext = ext' := equiv_path_extension ext ext'.
imports: HoTT.Basics HoTT.Types Equiv.PathSplit PathAny Cubical.DPath Cubical.DPathSquare Colimits.Coeq Colimits.MappingCylinder