∀ (x : List Char) (x_1 x_2 x_3 : String.Pos.Raw),
  String.Pos.Raw.extract.go₁ x x_1 x_2 x_3 =
    match x, x_1, x_2, x_3 with
    | [], x, x_4, x_5 => []
    | s@h:(c :: cs), i, b, e =>
      if i = b then String.Pos.Raw.extract.go₂ s i e else String.Pos.Raw.extract.go₁ cs (i + c) b e