15. The method of claim 13 wherein the penalty comprises a tree-structured l ∞ -norm.
15. The method of claim 13 wherein the penalty comprises a tree-structured l ∞ -norm. 16. The method of claim 15 wherein the tree-structured l ∞ -norm operates on a collapsed suffix trie in which any series of suffixes of increasing lengths which are always observed in the same context are collapsed into a single node.