Visitor pattern revisited in D

[deadalnix]

/!\ Shameless autopromotion incoming /!\

I have recently put some effort into exploring alternatives to visitor 
pattern and see what can be done in D. I ended up with a solution which 
is a real improvement compared to plein old visitor pattern and wanted 
to share this here.

I think this is short enough to be a good example to show what can be 
done with D capabilities.

http://www.deadalnix.me/2012/08/25/visitor-pattern-revisited-in-d/