∀ {α : Type u_1} {r : SetRel α α} (p : RelSeries r) (i : Fin (p.length + 1)),
  p.take i =
    { length := ↑i,
      toFun := fun x =>
        match x with
        | ⟨j, h⟩ => p.toFun ⟨j, ⋯⟩,
      step := ⋯ }