/* Modular exponentiation */
long xtoymn(x,y,n) long x,y,n; {
   long result;

   if (n<=0)  return 0;
   result = 1;

   while (y>0) {
     if (y & 1) {
       result = xbyymn(result,x,n);
     } 
     x = xbyymn (x,x,n) ;
     y = y>>1 ;
   }

   return result;
}