BNF

<expression> ::= <sum>
<sum>        ::= <product> |
                 <product> "+" <sum> |
                 <product> "-" <sum>
<product>    ::= <factor> |
                 <factor> "*" <product> |
                 <factor> "/" <product>
<factor>     ::= <number> | <symbol> |
                 "-" <factor> | "+" <factor> |
                 "(" <expression> ")"