eval s' = do
  st@(PegState s a w n c b p) <- get
  put $ PegState s' [] w n c b p
  force
  s'' <- getStack
  put st
  return s''
eval s' = do
  st@(PegState s a w n c b p) <- get
  put $ PegState s' [] w n c b p
  force
  s'' <- getStack
  put st
  return s''
eval s' = do
  st@(PegState s a w n c b p) <- get
  put $ PegState s' [] w n c b p
  force
  s'' <- getStack
  put st
  return s''