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Arithmetic {
  exp    = addExp  addExp    = addExp "+" mulExp  -- plus    | addExp "-" mulExp  -- minus    | mulExp  mulExp    = mulExp "*" priExp  -- times    | mulExp "/" priExp  -- divide    | priExp  priExp    = "(" exp ")"        -- paren    | number  number  (a number)    = number digit       -- rec    | digit}