-- | @replace a z@ changes the current element in the zipper
-- to the passed in value.  If there is no current element,
-- the zipper is unchanged.  If you want to add the element
-- in that case instead, use @insert a (delete z)@.
replace :: a -> Zipper a -> Zipper a
replace a (Zip ls (_:rs)) = Zip ls (a:rs)
replace :: a -> Zipper a -> Zipper a
replace a (Zip ls (_:rs)) = Zip ls (a:rs)
replace a (Zip ls (_:rs)) = Zip ls (a:rs)