Please write a function in Haskell which does the following, without using any prelude functions or importing any libraries. Make relevant helper functions where needed.

Given a binary tree return only the values "on the top line" 'On the top line" means only the ones that can be accessed by going only left or only right olus the root node. The list should be ordered from left to right. For example: The answer should be: [2,1,3,7] Note: No imports are allowed