6. The method according to claim 5 wherein the very small sets comprise keep-together sets including only a single document.
5. The method according to claim 1 and also comprising filling batches, separately for each of several clusters, including first combining keep-together sets other than very small sets 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 very small sets thereto, in the order determined by known urgency.