∀ {X : SSet} {n : ℕ} {Y : SSet} (f : X ⟶ Y) [CategoryTheory.Mono f] (x : X.obj (Opposite.op (SimplexCategory.mk n))),
  (f.app (Opposite.op (SimplexCategory.mk n)) x ∈ Y.nonDegenerate n) = (x ∈ X.nonDegenerate n)