Produce a query fragment suitable for selecting documents public.
		
		Now (no arguments), at a specific time (datetime argument), or relative to now (timedelta).

def only_published(cls, at=None):
		"""Produce a query fragment suitable for selecting documents public.
		
		Now (no arguments), at a specific time (datetime argument), or relative to now (timedelta).
		"""
		
		if isinstance(at, timedelta):
			at = utcnow() + at
		else:
			at = at or utcnow()
		
		pub, ret = cls.published, cls.retracted
		
		publication = (-pub) | (pub == None) | (pub <= at)
		retraction = (-ret) | (ret == None) | (ret > at)
		
		return publication & retraction