code: function distBBox(node, k, p, toBBox, destNode) {
      if (!destNode) destNode = createNode(null);
      destNode.minX = Infinity;
      destNode.minY = Infinity;
      destNode.maxX = -Infinity;
      destNode.maxY = -Infinity;
  
      for (var i = k, child; i < p; i++) {
          child = node.children[i];
          extend(destNode, node.leaf ? toBBox(child) : child);
      }
  
      return destNode;
  }
docs: min bounding rectangle of node children from k to p-1