function Obj(){}
Obj.prototype.x = '';

o = new Obj();

with(o){
	x = 'xxx';
}

trace('o.x is ' + o.x); //o.x is

o.x = 123;
with(o){
	x = 'xxx';
}

trace('o.x is ' + o.x); //o.x is xxx

with(button1){
	valueOf().enabled = false;
}