This is the most updated version of a talk presenting the result contained here. The talk was given in plenary session at Topology, Algebra, and Category in Logic -TACL- V, Marseille, 28$^{th}$ July. 2011
This is the most updated version of a talk presenting the result contained here. The talk was given in plenary session at Topology, Algebra, and Category in Logic -TACL- V, Marseille, 28$^{th}$ July. 2011