This article contains the best description: http://www.w3schools.com/e4x/default.asp
"E4X is a Web Standard"

This is actually pretty incredible - E4X is JavaScript manipulation over XML documents!!!

This means that web browsers (we already knew Flex supports it) can parse and programmatically access XML documents, actually XML documents can programmatically access other XML documents!!
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output: "This is so cool!!!"

other articles:
http://www.ibm.com/developerworks/webservices/library/ws-ajax1/
http://www.xml.com/pub/a/2007/11/28/introducing-e4x.html

(ps: Translation: Oh my gosh, European Computer Manufacturers Association Script is JavaScript and a web standard - ECMAScript for XML woohoo!)

I do enjoy contracts and some parts are just plain interesting - it makes one wonder where some of this comes from...