Stable topN?

[Xinok via Digitalmars-d]

I've managed to build a small laundry list the past couple days, 
so why not add another item? I've been tasked with implementing a 
"deterministic topN", but I also noticed that topN is lacking a 
stable implementation, so this seemed like an ideal opportunity 
to kill two birds with one stone. However, for me, it begged the 
question:

What exactly does stable topN imply?

A stable sort only implies that equal elements retain their 
original order. However, topN is a partitioning function whose 
pivot is unknown beforehand. To me, it would make more sense that 
not just equal elements but ALL elements retain their original 
order in their respective partitions.

However, to accomplish the above, the Nth element would need to 
be discovered before any other elements are reordered. This 
suggests making a full copy of the range to search for the Nth 
element. The unstable topN could be applied to the copy to 
discover the Nth element, then the stable partition function 
could finish the job with the correct pivot.

What do you all think? Do you agree with my definition of stable 
topN? Do you know of a better algorithm/approach for implementing 
this function?