4. The method of claim 1 wherein the one or more conversation sessions comprises at least one previous session between the first user and the dialog system, and not including the current session user input.
5. The method of claim 4 further comprising: utilizing one of a parse-tree based approach or a chunking-based approach to extract phrases used by the first user in a constraint specification and form a pool of examples of how to lexicalize constraints in the user input; and generalizing the constraints into phrase patterns by replacing constraints with corresponding semantic class names.