how to get top N distinct elements from range?

[bearophile]

Andrea Fontana:

> Maybe two versions (filter and cachedFilter) or a bool template 
> param?

In the meantime I have filed the problem, I hope Andrei will take 
a look:

http://d.puremagic.com/issues/show_bug.cgi?id=9674

Bye,
bearophile