String Constructor: self-contained re-assignment ??

I'm puzzled by this:

Why can't I re-assign a new value to a String instance using a custom constructor method? It seems that the following code should work, but it fails every time. (MX 2004)...

Code:

---

function extendString() {
  String.prototype.replaceValue = function(newValue:String) {
    this = newValue;
  };
}
extendString();
var myString = "this is the initial string value";
  myString.replaceValue("this is the changed string value");
  // myString should now have a different value, but it doesn't!
  //  the following works, but is NOT self-contained:
  //    myString = myString.replaceValue("this is the changed string value");

---

Any ideas? Essentially, I want to be able to alter the instance of String internally just like the behavior of the Array.splice() method.

+Q__