4. The method of claim 1 , wherein the following operations are performed on at least two documents in the index-word document list, in order of decreasing word similarity: for each of said at least two documents, setting one of said at least two documents as a first document D; choosing a second document D 2 to be the highest-lying document on the index-word document list which (i) has not been chosen before, and (ii) lies lower on the index-word document list than first document D; checking, for each second document D 2 lying lower on the index-word document list than first document D, whether the similarity S(D,D 2 ) has been previously calculated; for each second document D 2 for which S(D,D 2 ) has not been previously calculated, calculating a similarity S(D,D 2 ) of first document D to second document D 2 ; and enforcing a similarity threshold τ SIM by stopping said step of choosing second documents, and by stopping said step of calculating S(D,D 2 ) for any further second document D 2 , when a similarity between document D and some second document D 2 is less than the similarity threshold.
4. The method of claim 1 , wherein the following operations are performed on at least two documents in the index-word document list, in order of decreasing word similarity: for each of said at least two documents, setting one of said at least two documents as a first document D; choosing a second document D 2 to be the highest-lying document on the index-word document list which (i) has not been chosen before, and (ii) lies lower on the index-word document list than first document D; checking, for each second document D 2 lying lower on the index-word document list than first document D, whether the similarity S(D,D 2 ) has been previously calculated; for each second document D 2 for which S(D,D 2 ) has not been previously calculated, calculating a similarity S(D,D 2 ) of first document D to second document D 2 ; and enforcing a similarity threshold τ SIM by stopping said step of choosing second documents, and by stopping said step of calculating S(D,D 2 ) for any further second document D 2 , when a similarity between document D and some second document D 2 is less than the similarity threshold. 11. The method of claim 4 , wherein said step of calculating a similarity of document D to all other documents of said at least two documents with a lower word similarity rank, and stopping said calculation when a calculated similarity is less than a similarity threshold τ SIM , comprises: choosing τ SIM independently for each word in the index.