(public) return + if this > a, - if this < a, 0 if equal
function bnCompareTo(a) {
  var this_array = this.array;
  var a_array = a.array;

  var r = this.s-a.s;
  if(r != 0) return r;
  var i = this.t;
  r = i-a.t;
  if(r != 0) return r;
  while(--i >= 0) if((r=this_array[i]-a_array[i]) != 0) return r;
  return 0;
}