∀ {X Y : SSet} (f : X ⟶ Y) (x : X.obj (Opposite.op (SimplexCategory.mk 0)))
  (y : Y.obj (Opposite.op (SimplexCategory.mk 0))) (h : f.app (Opposite.op (SimplexCategory.mk 0)) x = y),
  (SSet.RelativeMorphism.ofSimplex₀ f x y h).map = f