ExtensionAlong@{a b p m} {A : Type@{a}} {B : Type@{b}} (f : A -> B) (P : B -> Type@{p}) (d : forall x:A, P (f x)) := (* { s : forall y:B, P y & forall x:A, s (f x) = d x }.
Require Import HoTT.Basics HoTT.Types. Require Import Equiv.PathSplit Homotopy.IdentitySystems. Require Import Cubical.DPath Cubical.DPathSquare. Require Import Colimits.Coeq Colimits.MappingCylinder.