Recover Cons in a newtype

recover-cons.hs

newtype Ledger a = Ledger { _lEntries :: Seq a }
    deriving (Eq,Show,Generic,Monoid)
makeLenses ''Ledger
instance Cons (Ledger a) (Ledger a) a a where
    _Cons = prism (\(a,s) -> over lEntries (review _Cons . (a,)) s) $
            \s -> case firstOf _Cons (view lEntries s) of
                    Nothing -> Left mempty
                    (Just (a,as)) -> Right (a,set lEntries as s)