expression grammar

@skip whitespace{

	expr ::= factor addexpr*;
	addexpr ::= '+' factor;
	factor ::= primary mulexpr*;
	mulexpr ::= '*' primary;
	primary ::= '(' expr ')' | number | variable;
	number ::= [0-9]+;
	variable ::= [a-z]+;
}

whitespace ::= [ ]+;