5. A method as claimed in claim 1 , comprising assigning each term to one of a plurality of subsets of terms depending on the frequency of occurrence of the term; and selecting, as a cluster attractor, the respective probability distribution of one or more terms from each subset of terms.
7. A method as claimed in claim 5 , wherein an entropy threshold is assigned to each subset, the method comprising selecting, as a cluster attractor, the respective probability distribution of one or more terms from each subset having an entropy that satisfies the respective entropy threshold.