[Haskell-cafe] Leaves of a Tree

Hello, Any recommendations for speeding up extracting the set of leaves from a tree? data Tree = Branch Tree Tree | Leaf Int deriving (Eq, Ord) My slow, naive function: leaves :: Tree -> Set Int leaves (Leaf n) = singleton n leaves (Branch left right) = union (leaves left) (leaves right) In my case, many of the branches in the tree are the same. I suspect the fixed point operation mentioned in the thread "speeding up fibonacci with memoizing" is applicable, but I'm having a tough time making the connection. -Tom