start = additive additive = left:multiplicative "+" right:additive { return
left + right; } / multiplicative multiplicative = left:primary "*" right:multiplicative { return left * right; } / primary primary = integer / "(" additive:additive ")" { return additive; } integer "integer" = digits:[0-9]+ { return
parseInt(digits.join(""), 10); } letter = [a-z]+