31. The invention as set forth in claim 25, further comprising: varying the contents of each of the data set context cluster and the history context cluster so as to achieve a minimum total cost.
25. A compression method for compressing a plurality of data sets, each of the data sets containing a history context and a data set context, the method comprising: clustering similar history contexts from a data set to form a history context cluster; and clustering similar data set contexts into a data set context cluster which share the same history contexts clusters.