Although in this example this region is a circle, in three dimensions it would be a sphere, and in
general we call it the candidate hypersphere.
The reason that this observation is so important is that it lets us prune which parts of the tree might
hold the true nearest neighbor. In particular, notice that this circle is entirely to the right of the
splitting hyperplane running vertically through the root of the tree. Consequently, any point to the
left of the root of the tree cannot possibly be in the candidate hypersphere, and consequently can't
be any better than our current guess. In other words, once we have a guess about where the nearest
neighbor is, we can start eliminating parts of the tree where the actual answer cannot be. This
general technique of searching a large space and pruning options based on partial results is called
branch-and-bound