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### Instructions
Write a function that applies a function in order to each element in the tree (see "in order tree walks").
Write a function that applies a given function `f`, **in order**, to each element in the tree.
### Notions
- [Tree Traversal](https://en.wikipedia.org/wiki/Tree_traversal)
### Expected function
Write a function that applies a function using a postorder walk to each element in the tree.
Write a function that applies a given function `f`, to each element in the tree using a postorder walk.
Write a function that applies a function using a preorder walk to each element in the tree.
Write a function that applies a given function `f` to each element in the tree using a preorder walk.