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