28. The system of claim 27, wherein said means for selecting a first training document to be a least-relevant document comprises: scoring means for determining an absolute relevance score of each of said plurality of training documents in said first set of training data, each absolute relevance score being indicative of an extent to which said training document includes preferred content; sorting means for sorting said plurality of training documents in said first set of training data on the basis of their absolute relevance scores to form a sorted list; and means for specifying a least relevant training document in said sorted list.
28. The system of claim 27, wherein said means for selecting a first training document to be a least-relevant document comprises: scoring means for determining an absolute relevance score of each of said plurality of training documents in said first set of training data, each absolute relevance score being indicative of an extent to which said training document includes preferred content; sorting means for sorting said plurality of training documents in said first set of training data on the basis of their absolute relevance scores to form a sorted list; and means for specifying a least relevant training document in said sorted list. 29. The system of claim 28, further comprising: ranking means for determining, on the basis of said absolute relevance score of said newly-received document and said absolute relevance scores of each of said plurality of training documents, a rank of said newly-received document relative to each of said plurality of training documents; and inserting said newly-received document into said sorted list according to said rank.