xpr =     const Zero <$> pTZero
     <|> (\_ _ v _ -> Succ v) <$> pTSuc  <*> pTLParen <*> pTVar <*> pTRParen  
     <|> (\_ _ v _ -> Pred v) <$> pTPred <*> pTLParen <*> pTVar <*> pTRParen  
pExpr =     const Zero <$> pTZero
     <|> (\_ _ v _ -> Succ v) <$> pTSuc  <*> pTLParen <*> pTVar <*> pTRParen  
     <|> (\_ _ v _ -> Pred v) <$> pTPred <*> pTLParen <*> pTVar <*> pTRParen
pExpr =     const Zero <$> pTZero
     <|> (\_ _ v _ -> Succ v) <$> pTSuc  <*> pTLParen <*> pTVar <*> pTRParen  
     <|> (\_ _ v _ -> Pred v) <$> pTPred <*> pTLParen <*> pTVar <*> pTRParen