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

level 1  +---+---+ (2 objects in NW node, recurse)
         |* *| * |
         +---+---+
         |*  |   |
         +---+---+

level 2  +-+-+---+ (1 object in each node, stop recursing)
         |*|*| * |
         +-+-+---+
         |*  |   |
         +---+---+

##</code><code>##

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