code: def only_published(cls, at=None):
  		
  		
  		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
docs: Produce a query fragment suitable for selecting documents public.
  		
  		Now (no arguments), at a specific time (datetime argument), or relative to now (timedelta).