18. The apparatus of claim 17 , wherein the SLU is configured to cease generating the updated probabilities that the categories of the taxonomy contain relevant documents when a change in the updated probabilities is less than a convergence threshold.
17. The apparatus of claim 13 , wherein the SLU is configured to determine the relevance of documents to the query by repeatedly generating the updated probabilities that the categories of the taxonomy contain relevant documents based on the previous updated probabilities that the categories of the taxonomy contain relevant documents and re-determining the updated relevance of documents to the query.