∀ {α : Type u_1} (xs : Array α) (i : ℕ) (h : i < xs.size) (h' : i + 1 < xs.size),
  InvImage (fun x1 x2 => x1 < x2) (fun x => PSigma.casesOn x fun xs i => PSigma.casesOn i fun i h => xs.size - i)
    ⟨xs.swap (i + 1) i h' h, ⟨i + 1, ⋯⟩⟩ ⟨xs, ⟨i, h⟩⟩