-- | @left z@ returns the zipper with the focus
-- shifted left one element.
left :: Zipper a -> Zipper a
left  (Zip (a:ls) rs) = Zip ls (a:rs)
left :: Zipper a -> Zipper a
left  (Zip (a:ls) rs) = Zip ls (a:rs)
left  (Zip (a:ls) rs) = Zip ls (a:rs)