∀ {T : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} T] (X : T × T)
  (Y : CategoryTheory.StructuredArrow X (CategoryTheory.Functor.diag T)),
  ((CategoryTheory.StructuredArrow.ofDiagEquivalence.functor X).obj Y).right.left.as = PUnit.unit