Add tree structure generation

src/Dodge/RandomHelp.hs

@@ -33,6 +33,7 @@ takeNMore :: RandomGen g => Int -> ([a],[a]) -> State g ([a],[a])
 takeNMore n p = foldr (const ((=<<) takeOneMore)) (return p) [1..n]
 
 takeN :: RandomGen g => Int -> [a] -> State g [a]
+takeN 0 xs = return []
 takeN i xs = fmap fst $ takeNMore i ([],xs)
 
 -- to randomly shuffle a list