10. The probabilistic clustering system as set forth in claim 8 , wherein the classes are organized into a hierarchy of classes with documents assigned to leaf classes of the hierarchy, the user interface is configured to receive a user selection to merge two or more classes to be merged, said two or more classes having a common immediate ancestor class, and the processor is configured to algebraically combine probabilistic model parameters characterizing the two or more classes to be merged to generate probabilistic model parameters characterizing a merged class.
10. The probabilistic clustering system as set forth in claim 8 , wherein the classes are organized into a hierarchy of classes with documents assigned to leaf classes of the hierarchy, the user interface is configured to receive a user selection to merge two or more classes to be merged, said two or more classes having a common immediate ancestor class, and the processor is configured to algebraically combine probabilistic model parameters characterizing the two or more classes to be merged to generate probabilistic model parameters characterizing a merged class. 11. The probabilistic clustering system as set forth in claim 10 , wherein the merged class is one of (i) a substitute class that replaces the two or more classes to be merged and (ii) a new ancestor class that is inserted between the two or more classes to be merged and a common ancestor of the two or more classes to be merged.