Past exam of the year 1 of the computer science course of the University of Oxford / 2018 / Functional programming, design and analysis of algorithms / 1 / g

Include each child itself, followed by its proper subtrees, while excluding the current root:

proper :: Tree a -> [Tree a]
proper (One _)   = []
proper (Two l r) = (l : proper l) ++ (r : proper r)

Again, occurrences matter: a child with the same value as another child is not removed. The definition is a structural recursion, but its right-hand side uses the original children as well as their recursively computed results.