∀ {X Y : SSet} {A : X.Subcomplex} {B : Y.Subcomplex} {φ : A.toSSet ⟶ B.toSSet} (self : SSet.RelativeMorphism A B φ),
  CategoryTheory.CategoryStruct.comp A.ι self.map = CategoryTheory.CategoryStruct.comp φ B.ι