(Article initially published in
December 1992 in the number 144 of the French
magazine "Le monde inconnu"
("the unknown world") and slightly
altered here by the author).
(Article initially published in
march 1993 in the number 147 of the French
magazine "Le monde inconnu"
("the unknown world") and altered here
by the author).
(Summary and revision by the author of an article initially
published in September 1992 in the number 141 of
the French magazine "Le monde
inconnu" ("the unknown
world"))