code: function bnpDAddOffset(n,w) {
    var this_array = this.array;
    while(this.t <= w) this_array[this.t++] = 0;
    this_array[w] += n;
    while(this_array[w] >= BI_DV) {
      this_array[w] -= BI_DV;
      if(++w >= this.t) this_array[this.t++] = 0;
      ++this_array[w];
    }
  }
docs: (protected) this += n << w words, this >= 0