function setOnChange(id) {
	document.getElementById(id).onchange = function() {
		var select = this.selectedIndex;
		if (select != -1) {
			var count = this.childElementCount;
			for (i=0; i < select-1; i++) {
				setOptionDisabled(id, i);
			}
			for (j=select; j < count; j++) {
				setOptionDisabled(id, j);
			}
			setOptionEnabled(id, select);
			if (select != 0) {
				setOptionEnabled(id, select-1);
				if (this.options[select-1].selected) {
					if (select != count-1) setOptionDisabled(id, select+1);
				}
			}
			if (select != count-1) {
				setOptionEnabled(id, select+1);
				if (this.options[select+1].selected) {
					if (select != 0) setOptionDisabled(id, select-1);
				}
			}
		}
		else resetOnChange(id)
	};
}
function resetOnChange(id) {
	var count = document.getElementById(id).childElementCount;
	for (i=0; i < count; i++) setOptionEnabled(id, i);
}
function setOptionDisabled(id, i) {
	document.getElementById(id).options[i].disabled = true;
}
function setOptionEnabled(id, i) {
	document.getElementById(id).options[i].disabled = false;
}