    (_, (rows, cols)) = bounds grid
    go !acc (i, j) = if i > rows then acc else go acc' (i + down, j + right)
      where
        acc' = acc + case grid ! (i, j `mod` (cols + 1)) of Tree -> 1; Open -> 0

------------ PART B ------------
partB :: Input -> OutputB
partB grid =
  foldMap