Idempotent partition around median of 5?

[Ivan Kazmenko via Digitalmars-d]

On Saturday, 6 February 2016 at 07:06:27 UTC, Ivan Kazmenko wrote:
> On Saturday, 6 February 2016 at 00:59:17 UTC, Andrei 
> Alexandrescu wrote:
>> On 02/05/2016 06:36 AM, Ivan Kazmenko wrote:
>>> Another interesting task would be to make the function 
>>> stable, but I don't see how it is possible with such flat 
>>> structure.
>>
>> Under what circumstances isn't your function unstable? -- 
>> Andrei

The code I used to check stability:
http://dpaste.dzfl.pl/2b950cb3e5d8

The "y" and "n" at end of lines are "yes"/"no", as in 
"stable"/"unstable".