double-spending ecash

Double spending using ecash isn't possible, but why so? Is it because of the verification protocol? Or is it an implication of the isueing intricacies? And how about tracing, is it merely unfeasable that ecash spending can be traced, or is it mathamatically impossible to trace? Any explanations or pointers would be extremely helpful. ------------------------------------------------------------------------------- Patiwat Panurach Whatever you can do, or dream you can, begin it. eMAIL: pati@ipied.tu.ac.th Boldness has genius, power and magic in it. m/18 junior Fac of Economics -Johann W.Von Goethe -------------------------------------------------------------------------------