Lysxia

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)