+ ysize = length (head fvs) - 1
+ xsize = length (head $ head fvs) - 1
+
+
+-- | Takes a grid and a position as an argument and returns the cube
+-- centered on that position. If there is no cube there (i.e. the
+-- position is outside of the grid), it will return 'Nothing'.
+cube_at :: Grid -> Int -> Int -> Int -> Maybe Cube
+cube_at g i j k
+ | i < 0 = Nothing
+ | j < 0 = Nothing
+ | k < 0 = Nothing
+ | i >= length (cubes g) = Nothing
+ | j >= length ((cubes g) !! i) = Nothing
+ | k >= length (((cubes g) !! i) !! j) = Nothing
+ | otherwise = Just $ (((cubes g) !! i) !! j) !! k
+
+
+-- | Takes a 'Grid', and returns all 'Cube's belonging to it that
+-- contain the given 'Point'.
+find_containing_cubes :: Grid -> Point -> [Cube]
+find_containing_cubes g p =
+ filter contains_our_point all_cubes
+ where
+ all_cubes = flatten $ cubes g
+ contains_our_point = flip contains_point p