how to get top N distinct elements from range?

[Ivan Kazmenko]

>>> I wonder if exists a way to get top n distinct elements from 
>>> a range (infinite too!)
>>
>> It's impossible to do that for infinite ranges
>
> Why?
>
> sequence!"n*2".myTopDistinct!"a==b"(3);
>
> will give [2,4,6]

I don't quite get it, do you want the least n elements?  What 
result would you want if the sequence was the following:

sequence!"-n*2".myTopDistinct!"a==b"(3);

The sequence goes like [-2, -4, -6, -8, -10, ...] ad infinitum.

To clarify a doubt, do you mean "top" as in "front", or "top" as 
the "greatest" or "least" in terms of comparison?  Or maybe "the 
first n distinct elements found"?  If so, why "sort" in the 
original post?  It could bring middle elements to front.