hbal :: a -> Int -> [Tree a]
hbal x n = filter is_hbal [t | t <- all_trees_and_smaller n x, nnodes t == n]
hbal :: a -> Int -> [Tree a]
hbal x n = filter is_hbal [t | t <- all_trees_and_smaller n x, nnodes t == n]
hbal x n = filter is_hbal [t | t <- all_trees_and_smaller n x, nnodes t == n]