Arithmetic {
  Exp
    = AddExp

  AddExp
    = AddExp "+" MulExp  -- plus
    | AddExp "-" MulExp  -- minus
    | MulExp

  MulExp
    = MulExp "*" PriExp  -- times
    | MulExp "/" PriExp  -- divide
    | PriExp

  PriExp
    = "(" Exp ")"  -- paren
    | number

  number
    = digit+
}