Andrew Coppin wrote:

trees :: [Int] -> [Tree]
trees = map fst . (\ts -> all_trees 1 (2 * length ts) ts) . map Leaf

all_trees :: Int -> Int -> [Tree] -> [(Tree,[Tree])]
all_trees n m ts
| n > m = []
| otherwise = pick ts ++ sub_trees n m ts

sub_trees :: Int -> Int -> [Tree] -> [(Tree,[Tree])]
sub_trees n m ts = do
let n2 = n * 2
(t0,ts0) <- all_trees n2 m ts
(t1,ts1) <- all_trees n2 m ts0
return (Branch t0 t1, ts1)

Idiot...

The size of the deepest possible balanced tree with N leaves is log2 N. The deepest possible unbalanced tree has N nodes!

For small N, it doesn't matter too much. But as N gets larger, the difference becomes... uh... large? (!)

*sigh* I hate being wrong. :-(