popLabel :: Monad m => MkPNMonad m value ()
    popLabel = state op
        where op mkpn = ((), mkpn')
                where mkpn' = mkpn { mkpnLabelStack = tail $ mkpnLabelStack mkpn }
popLabel :: Monad m => MkPNMonad m value ()
popLabel = state op
        where op mkpn = ((), mkpn')
                where mkpn' = mkpn { mkpnLabelStack = tail $ mkpnLabelStack mkpn }
popLabel = state op
        where op mkpn = ((), mkpn')
                where mkpn' = mkpn { mkpnLabelStack = tail $ mkpnLabelStack mkpn }