code: function nextDeep(stream) {
      if (!stream.cursor) return false;
      var cursor = stream.cursor;
      cursor.x++;
      var bbox = stream.bboxes[cursor.z];
      if (cursor.x > bbox.maxX) {
          cursor.x = bbox.minX;
          cursor.y++;
      }
      if (cursor.y > bbox.maxY) {
          cursor.z++;
          if (cursor.z > stream.maxzoom) {
              stream.cursor = false;
              return false;
          }
          bbox = stream.bboxes[cursor.z];
          cursor.x = bbox.minX;
          cursor.y = bbox.minY;
      }
      return true;
  }
docs: Increment a tile cursor to the next position, descending zoom levels until maxzoom is reached.