BNF2

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