<expr> ::= <term> <expr_tail>

<expr_tail> ::= + <term> <expr_tail>
              | - <term> <expr_tail>
              | <empty>

<term> ::= <factor> <term_tail>

<term_tail> ::= * <factor> <term_tail>
              | / <factor> <term_tail>
              | <empty>

<factor> ::= "(" <expr> ")"
           | Num

Identifier ::= <character2> <name>

name ::= <name>
       | <character1>
       | <empty>

<character1> ::= <letter> | <digit> | "_"

<character2> ::= <letter> | "_"

<letter> ::= "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" | "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" | "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z" | "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z"

<digit> ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"

<empty> ::= ""