haveConstant = true
	case '(':
		// Could be parenthesized expression or (R). Must be something, though.
		tok := p.next()
		if tok.ScanToken == scanner.EOF {
			p.errorf("missing right parenthesis")
			return
		}
		rname := tok.String()
		p.back()
		haveConstant = !p.atStartOfRegister(rname)
		if !haveConstant {
			p.back() // Put back the '('.
		}
	}
	if haveConstant {
		p.back()