The submission had four reviewers. The scores of the original reviews were somewhat varied. The reviewers considered the problem setting generally well motivated and interesting, and the authors make solid progress with interesting results. However, questions were raised about the appropriateness of some particular assumptions of the model. There is some empirical evaluation, which is good, but in its present stage it's much too limited. All the reviewers also pointed out shortcomings in the presentation, some of them serious. The authors' reply addressed the questions about the model and satisfied the reviewers partly, but not entirely. The authors also clarified some unclear points in the current version and promised more work on the presentation in a revised version, and more extensive experiments. As a result of the authors' reply and lively discussion among the reviewers, some of the reviewers raised their evaluation, and they are now unanimously proposing accepting the paper. However, some reservations remain. Overall, I consider the submission somewhat above the acceptance threshold.