Use grid when creating block debris

src/ListHelp.hs

@@ -9,6 +9,7 @@ module ListHelp
     , errorHead
     , foldPairs
     , loopPairs
+    , slideWindow
     ) where
 import Data.List
 
@@ -61,3 +62,6 @@ safeUncons _ = Nothing
 errorHead :: String -> [a] -> a
 errorHead _ (x:_) = x
 errorHead s [] = error s
+
+slideWindow :: Int -> [a] -> [[a]]
+slideWindow n = foldr (zipWith (:)) (repeat []) . take n . tails