Mapping over last breadcrumb

Is it possible to map over last breadcrumb using lens?

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeOperators #-}
import Control.Lens
import Control.Zipper

data A = AA { _aa :: A }
       | AB { _ab :: B }
       deriving (Show)

data B = B deriving (Show)

makeLenses ''A
makeLenses ''B

main :: IO ()
main = do
    let a = AA $ AB $ B

        z :: Top :>> A
        z = zipper a

        zAA :: Maybe (Top :>> A :>> A)
        zAA = z & within aa

        zAB :: Maybe (Top :>> A :>> B)
        zAB = z & within (aa . ab)
    return ()

I can move from Top :>> A either to Top :>> A :>> A and Top :>> A :>> B.

Having ab lens, is it possible to move from Top :>> A :>> A (zAA) to Top :>> A :>> B (zAB), without using upward - just mapping with lens over last breadcrumb?

ekmett / zippers

Mapping over last breadcrumb #6