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level 0  +-------+  (4 objects in the node, recurse)
         |* *  * |
...
         +-+-+---+
         |*  |   |
         +---+---+

 * If a node's bounding box is outside the N-unit circle,
   stop.
...
   objects inside it and stop.
 * Otherwise (intersects and isn't a leaf node), recurse on
   the node's four children.