39. The system of claim 38 , wherein the processor is further configured to execute operations including: building the hierarchical structure using a decision tree.
39. The system of claim 38 , wherein the processor is further configured to execute operations including: building the hierarchical structure using a decision tree. 40. The system of claim 39 , wherein the processor is further configured to execute operations to classify the plurality of data files within the hierarchical structure using the decision tree, wherein the decision tree employs a criterion sensitive to an absence of the key terms in the plurality of data files, and wherein each node of the hierarchical structure is associated with the homogenous sub-group.