I'll probably never understand Internet Explorer

I was coding to solve eval re-definition problem ... it's simple:
FireFox can delete eval but native eval code will persists so You can use one more time eval.

IE7 can delete eval ... and after this operation, eval will be not usable ...

What's the awesome behaviour?
IE7 has another method to evaluate code, execScript ... and It doesn't accept this code

execScript = function(){};

Cool? It seems that I can believe on execScript and delte eval only on FireFox ... why not?

Because IE7 accepts this code

window.execScript = function(){};

So bye bye execScript, You've been defined ... but the coolest thing is that after that code, You can't do:

delete window.execScript;

It's amazing IE7 deveolpers, thank you one more time!