chore(day6): add todo for cleaning

app/Day6.hs

@@ -7,6 +7,9 @@ import qualified Data.IntMap as M
 import Data.List (foldl')
 import System.Environment (getArgs)
 
+-- TODO: solve part 2 by transposing the matrix before parsing the
+-- numbers.
+
 main :: IO ()
 main = do
   input <- readFile . head =<< getArgs