code: function bnCompareTo(a) {
    var r = this.s - a.s;
    if (r != 0) return r;
    var i = this.t;
    r = i - a.t;
    if (r != 0) return this.s < 0 ? -r : r;
    while (--i >= 0)
      if ((r = this[i] - a[i]) != 0) return r;
    return 0;
  }
docs: (public) return + if this > a, - if this < a, 0 if equal