-- | @reversez z@ returns the zipper with the elements in
-- the reverse order.  O(1).  The cursor is moved to the
-- previous element, so if the cursor was at the start,
-- it's now off the right end, and if it was off the
-- right end, it's now at the start of the reversed list.
reversez :: Zipper a -> Zipper a
reversez (Zip ls rs) = Zip rs ls
reversez :: Zipper a -> Zipper a
reversez (Zip ls rs) = Zip rs ls
reversez (Zip ls rs) = Zip rs ls