This invisible foot idea is quite interesting because it has some link with the fear of many of those who are skeptical about tradable permits: They say: "We are in fact granting extra rights to polluters" The standard response: "Not at all! We are simply restricting their freedom by imposing restrictions on emissions. But we do so with the help of a market mechanism" The invisible foot idea as a reply to this standard response: "in a dynamic perspective, tradable permits generate an incentive to generate new negative externalities" Does this make sense? What are the references to the Hunt and the D'arge & M?daille papers? Dr. Axel Gosseries Ramalho