ed tab to the given index, or to the end if the
-- index is not specified.
moveTabE :: Maybe Int -> EditorM ()
moveTabE Nothing  = do
    count <- uses tabsA PL.length
    tabsA %= fromJust . PL.moveTo (pred count)
moveTabE (Just n) =
moveTabE :: Maybe Int -> EditorM ()
moveTabE Nothing  = do
    count <- uses tabsA PL.length
    tabsA %= fromJust . PL.moveTo (pred count)
moveTabE Nothing  = do
    count <- uses tabsA PL.length
    tabsA %= fromJust . PL.moveTo (pred count)