∀ {α : Type u_1} (_x : (_ : List α) ×' (α → α → Bool)),
  List.mergeSort._unary _x =
    PSigma.casesOn _x fun xs le =>
      match xs, le with
      | [], x => []
      | [a], x => [a]
      | a :: b :: xs, le =>
        have lr := List.MergeSort.Internal.splitInTwo ⟨a :: b :: xs, ⋯⟩;
        have this := ⋯;
        have this := ⋯;
        (List.mergeSort._unary ⟨↑lr.1, le⟩).merge (List.mergeSort._unary ⟨↑lr.2, le⟩) le