Many thanks to Henri Cohen who started writing this program. 
After this program is available, many people pointed out to 
me (mathematical as well as programming) bugs :
B. Poonen, E. Schaefer, C. Stahlke, M. Stoll, F. Villegas.
So thanks to all of them. Thanks also go to Ph. Depouilly
who help me to compile the program. 


