13. Apparatus for searching a corpus of documents, comprising: a memory, adapted to store an identification of a set of reference documents in the corpus pertinent to a predefined knowledge domain; and a search processor, which responsive to receiving a first query as input, is adapted to search the corpus using the set of reference documents to find one or more of the documents in the corpus that contain information in the domain relevant to the first query, and to add at least one of the found documents to the set of reference documents stored in the memory for use in searching the corpus for information in the domain relevant to a second, subsequent query, which is substantially different from the first query, wherein the processor is adapted to receive the documents found to contain the information relevant to the query and to rank the found documents by assessing links between the found documents and the reference documents.
13. Apparatus for searching a corpus of documents, comprising: a memory, adapted to store an identification of a set of reference documents in the corpus pertinent to a predefined knowledge domain; and a search processor, which responsive to receiving a first query as input, is adapted to search the corpus using the set of reference documents to find one or more of the documents in the corpus that contain information in the domain relevant to the first query, and to add at least one of the found documents to the set of reference documents stored in the memory for use in searching the corpus for information in the domain relevant to a second, subsequent query, which is substantially different from the first query, wherein the processor is adapted to receive the documents found to contain the information relevant to the query and to rank the found documents by assessing links between the found documents and the reference documents. 15. The apparatus according to claim 13 , wherein the processor is adapted to add to the corpus at least the document having the highest ranking.