Static operator overloads are possible why?

[Frits van Bommel]

Giles Bathgate wrote:
>> DFL (very .Net like form library for D) does this. 
> 
> Is DFL open source?

http://www.dprogramming.com/dfl.php
It says the download contains source, but doesn't mention the license (I 
didn't download it). Though I guess the forum being hosted on 
dsource.org might be a clue that it's probably open source.