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