Expression  ::= AdditionExpression

AdditionExpression  ::=
    MultiplicationExpression
        | AdditionExpression '+' MultiplicationExpression
        | AdditionExpression '-' MultiplicationExpression

MultiplicationExpression    ::=
    Term
        | MultiplicationExpression '*' Term
        | MultiplicationExpression '/' Term

Term    ::=
    Number
        | '(' AdditionExpression ')'

Number  ::=
    [+-]?[0-9]+(.[0-9]+)?

function parse_addition_expression() {
    num = parse_multiplication_expression()
    while (has_token()) {
        get_token()
        if (current_token_type == PLUS)
            num += parse_multiplication_expression()
        else if (current_token_type == MINUS)
            num -= parse_multiplication_expression()
        else {
            unget_token()
            return num
        }
    }
    return num
}