Dear Colleagues, sorry for attaching yesterday a couple of files because of lack of my attension ! I know that for some of your attached files create difficulties. I did not check that replying to David, I answered to the whole SIG community. I hope you excuse me for this involuntary inconvenience. Anyhow, I hope that this presentation of my point of view, in parts slightly different from that widely considered, was useful for understanding the problem, specially for those who do not work in this field directly. With best regards, and have a nice week-end, Sasha
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