[Haskell-cafe] Branches of a binary tree

I am writing some code to find peaks in molecular spectra. I represent a spectrum as a list of numbers (intensities) and build a binary search tree of the positions in the list (x-values) sorted by intensity. Peaks in the spectrum appear as branches of the tree. My task is to return branches of a binary tree that contain no other branches larger than some size. I wrote this function to test whether a tree can be classified as a peak. I suspect it's very inefficient, and it doesn't seem idiomatic to me either. I'm looking for suggestions to improve it: -- |Returns True if the tree can be classified as a peak. Peaks are -- qualified by not having any two branches with a size greater than n. isPeak :: Int -> Tree a -> Bool isPeak _ Tip = True isPeak n (Node _ l r) | (ltn l) && (ltn r) = True | (ltn l) && (gtn r) = isPeak n r | (gtn l) && (ltn r) = isPeak n l | (gtn l) && (gtn r) = False | otherwise = error "isPeak: no matching condition in guard" where gtn = (>=n) . size ltn = ( Tree a -> a -> Tree a insert Tip x = Node x Tip Tip insert (Node y l r) x | x <= y = Node y (insert l x) r | otherwise = Node y l (insert r x) growTree :: Ord a => [a] -> Tree a growTree = foldl insert Tip size :: Tree a -> Int size Tip = 0 size (Node x l r) = 1 + (size l) + (size r) flatten :: Tree a -> [a] flatten Tip = [] flatten (Node x l r) = flatten l ++ [x] ++ flatten r atLeast :: Int -> [a] -> Bool atLeast 0 _ = True atLeast _ [] = False atLeast n (_:ys) = atLeast (n-1) ys peaks :: Int -> Tree a -> [[a]] peaks _ Tip = [] peaks n t@(Node _ l r) | isPeak n t = [flatten t] | otherwise = peaks n l ++ peaks n r -- Stolen from http://haskell.org/haskellwiki/Blow_your_mind sortIndicies ys = map fst $ L.sortBy (comparing snd) $ zip [0..] ys where comparing f x y = compare (f x) (f y)