∀ {α : Type u_1} (a b : α) (xs : List α) (le : α → α → Bool),
  (↑(List.MergeSort.Internal.splitInTwo ⟨a :: b :: xs, ⋯⟩).2).length = (xs.length + 1 + 1) / 2 →
    InvImage (fun x1 x2 => x1 < x2) (fun x => PSigma.casesOn x fun xs le => xs.length)
      ⟨↑(List.MergeSort.Internal.splitInTwo ⟨a :: b :: xs, ⋯⟩).2, le⟩ ⟨a :: b :: xs, le⟩