 Solution

ID: 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/solution

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.

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