// expr = term | term ('+' | '-' | '|' | '^') term.
func (p *Parser) expr() uint64 {
	value := p.term()
	for {
		switch p.peek() {
		case '+':
			p.next()
			value += p.term()
		case '-':
			p.next()
			value -= p.term()
		case '|':
			p.next()
			value |= p.term()
		case '^':
			p.next()
			value ^= p.term()
		default:
			return value
		}