popStack :: Peg Value
popStack = do PegState s a w n c b p <- get
              guard . not $ null s
              put $ PegState (tail s) a w n c b p
              return $ head s
popStack :: Peg Value
popStack = do PegState s a w n c b p <- get
              guard . not $ null s
              put $ PegState (tail s) a w n c b p
              return $ head s
popStack = do PegState s a w n c b p <- get
              guard . not $ null s
              put $ PegState (tail s) a w n c b p
              return $ head s