13. The system of claim 12 , wherein at least a subset of the clusters each have a single document; and wherein the cluster sequences corresponding to the subset of the clusters that each have a single document precede, within the compressed sequence of tokens, the cluster sequences corresponding to respective clusters that each have a plurality of documents.
12. The system of claim 10 , wherein generating the compressed sequence of tokens comprises: encoding each cluster of documents into a respective cluster sequence of tokens, each cluster sequence of tokens including a set of document reconstruction data for reconstructing documents in the corresponding cluster; and concatenating the respective cluster sequences of tokens into the compressed sequence of tokens.