function check_aantal(fld) {
    found=!isNaN(fld.value);
    if (!found) fld.value = 1;
    if (fld.value < 1) fld.value = 1;
}

function add_to_aantal (fld) {
	check_aantal(fld);
	fld.value = parseInt(fld.value) + 1;
}

function subtract_from_aantal (fld) {
	check_aantal(fld);
	if (fld.value > 1) fld.value = parseInt(fld.value) - 1;
}