docs: function Maze(width, height) {
      if (width < 0 || height < 0)
          throw new Error('invalid size: ' + width + 'x' + height);
      this._width = width;
      this._height = height;
      this._blockWidth = ((width+1)+15) >> 4;
      this._grid = new Array(this._blockWidth * (height + 1));
      for (var i = 0; i < this._blockWidth * (height + 1); i ++)
          this._grid[i] = 0;
  }