AS3 singleton implementations

Question

I have seen so many implementations of singletons around, and i just want a singleton that

1.- instances on the first call 2.- instances only once (duh)

So

Answer 1

example 2 but with a twist since you should allow new Singleton() to be called at least once and I don't like instantiating things until I need them, thus the first call to instance() actually creates the instance... subsequent calls grab the original.

EDIT: Sowed how it can also allow for if you call

var singleton:Singleton = new Singleton();

it will work... but all future tries will throw the error and force use of the getInstance() method

public final class Singleton{
    private static var _instance:Singleton;

    public function Singleton(){
        if(_instance){
            throw new Error("Singleton... use getInstance()");
        } 
        _instance = this;
    }

    public static function getInstance():Singleton{
        if(!_instance){
            new Singleton();
        } 
        return _instance;
    }
}