∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] {ι : Type w}
  {I : CategoryTheory.Limits.MultispanIndex (CategoryTheory.Limits.MultispanShape.prod ι) C} (self : I.SymmStruct)
  (i : ι), I.fst (i, i) = I.snd (i, i)