∀ {R : Type u} [inst : Ring R] {M N O : ModuleCat R} (f : M ⟶ N) (g : N ⟶ O) (x : ↑M),
  (CategoryTheory.ConcreteCategory.hom (CategoryTheory.CategoryStruct.comp f g)) x =
    (CategoryTheory.ConcreteCategory.hom g) ((CategoryTheory.ConcreteCategory.hom f) x)