Return type of BinarySearchTree.predecessor is necessary?

[bbvch13531]

Brief Intro

predecessor, successor function in BinarySearchTree

public func predecessor() -> BinarySearchTree<T>?
public func successor() -> BinarySearchTree<T>?

Is BinarySearchTree<T>? necessary? I think BinarySearchTree? will fine.
If BinarySearchTree<T>? is necessary, would you please let me know the reason why?
If not, I take this issue and make PR.
Thanks 😄 .

More Details

[kelvinlauKL]

Probably. The author may have forgotten the briefer version. Feel free to make the correction if the compiler agrees with you 👍