Return the directory names to preserve under the KLogN purge strategy.

def strategyKLogN(kls, n, k=4):
		"""Return the directory names to preserve under the KLogN purge strategy."""
		assert(k>1)
		s = set([n])
		i = 0
		
		while k**i <= n:
			s.update(range(n, n-k*k**i, -k**i))
			i += 1
			n -= n % k**i
		
		return set(map(str, filter(lambda x:x>=0, s)))