program :=  expr
expr    :=  expr "+" term | expr "-" term | term
term    :=  term "*" number | term "/" number | number | "(" expr ")"
number  :=  digits
digits  :=  digits digit | digit
digit   := "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"