∀ {C : Type u₂} [inst : CategoryTheory.Category.{v₂, u₂} C] {I : Type u₁} {J : Type u₁'} (f : J → C) (g : I → J)
  (X : CategoryTheory.Discrete I),
  (CategoryTheory.Discrete.functorComp f g).inv.app X = CategoryTheory.CategoryStruct.id (f (g X.as))