Pit the damsons and mince them (or put them through the meat grinder or puree in the blender or food processor), retaining as much of the juice as possible. Add a little water to the minced damsons and the juice and slowly bring to a boil' reduce the fruit pulp over low heat for several hours, but do not stir or the damson pulp is quite likely to scorch on the bottom. When the pulp has thickened somewhat, it will have to be stirred continuously for several hours, until it is literally thick enough that a spoon will stand up in it. It may sputter and bubble during this stage. (In former days, it was usual for several families to convene in the village washhouse for a cooperatove 'Zwetschgenmus' cooking session, with story-telling and other impromptu entertainments to give encouragement to the stirrers.) When the damson cheese has sufficiently thickened, transfer it to stoneware crocks and bake in the oven until a dry crust has formed on top. The damson cheese will keep better if a piece of parchment paper that has been soaked in rum is placed on top of this crust before the crock is sealed with plastic wrap (formerly a piece of linen or parchment was used). A good imported product is available on the American market under the name of 'Pflaumenmus' (pronounced Flaumenmoose).