"One feature of the voucher plan that has aroused particular concern is the possibility that parents could and would "add on" to the vouchers. If the voucher were for, say, $1,500, a parent could add another $500 to it and send his child to a school charging $2,000 tuition. Some fear that the result might be even wider differences in educational opportunities than now exist because low-income parents would not add to the amount of the voucher while middle-income and upper-income parents would supplement it extensively."