Enumerating BinTrees by depth

BinTrees.hs

import qualified Control.Monad.WeightedSearch as W
import Data.List (sortBy)
import Data.Ord (comparing)
import Control.Applicative

data BTree = Leaf | Branch BTree BTree deriving (Show, Eq)

-- I can lazily list all the btrees
btrees :: W.T Integer BTree
btrees = pure Leaf <|> W.weight 1 (Branch <$> btrees <*> btrees)

-- Now, let's consider this function
depth :: BTree -> Integer
depth Leaf = 0
depth (Branch b1 b2) = 1 + max (depth b1) (depth b2)

-- I'd like to generate the btrees in an order such that
-- `test btrees == True`, where":
test :: W.T Integer BTree -> Bool
test ts = ls == sortBy (comparing depth) ls
  where ls = take 1000 $ W.toList ts

-- that is, trees should be ordered by depth