6. The method according to claim 1 , further comprising: filling batches, separately for each of the clusters, including combining first keep-together subsets other than small keep-together subsets into batches; and subsequently passing over the batches in an order determined by known urgency and enlarging at least some of the batches by adding the small keep-together subsets thereto, in the order determined by the known urgency.
6. The method according to claim 1 , further comprising: filling batches, separately for each of the clusters, including combining first keep-together subsets other than small keep-together subsets into batches; and subsequently passing over the batches in an order determined by known urgency and enlarging at least some of the batches by adding the small keep-together subsets thereto, in the order determined by the known urgency. 7. The method according to claim 6 , wherein a small keep-together subset comprises only a single document.