∀ {α : Type u_1} (cut : α → Ordering) (f : α → α) (t : Batteries.RBNode α),
  Batteries.RBNode.modify cut f t =
    match Batteries.RBNode.zoom cut t with
    | (Batteries.RBNode.nil, snd) => t
    | (Batteries.RBNode.node c a x b, path) => path.fill (Batteries.RBNode.node c a (f x) b)