6. The method of claim 1 , wherein during step (b), each of the context trees includes frequent itemset information and propriety evaluation numeric information, the propriety evaluation numeric information includes a cohesion degree measurement value, a continuous power measurement value, and a boundary purity measurement value, the cohesion measurement value is an evaluation measure for a simultaneous generation rate of items included in the context and has a measurement value for each context, the continuous power measurement value is a generation rate of a child context under a condition where a parent context is generated, and the boundary purity measurement value digitalizes the purity of the contexts found within the determined boundary.
6. The method of claim 1 , wherein during step (b), each of the context trees includes frequent itemset information and propriety evaluation numeric information, the propriety evaluation numeric information includes a cohesion degree measurement value, a continuous power measurement value, and a boundary purity measurement value, the cohesion measurement value is an evaluation measure for a simultaneous generation rate of items included in the context and has a measurement value for each context, the continuous power measurement value is a generation rate of a child context under a condition where a parent context is generated, and the boundary purity measurement value digitalizes the purity of the contexts found within the determined boundary. 7. The method of claim 6 , wherein the cohesion degree (CD) is defined by CD = 1 - max ⁡ ( i k ) - min ⁡ ( i k ) { item cnt ⁡ ( C x ) - 1 } × δ (where, itemcnt is the number of items belonging to the context C x and i k is one of a set {i 1 , i 2 , . . . , i k } of items included in the context Cx, and 1≦k≦n).