int yylex()
{
    int car;
    car = getchar();
    if (car == EOF)
	return 0;
    if (isdigit(car)) {
	yylval = car - '0';
	return NB;
    }
    switch (car) {
    case '+':
	return PLUS;
    case '*':
	return FOIS;
    case '!':
	return FIN;
    }

}