<p>In <a href="https://github.com/mlpack/mlpack/pull/760#discussion_r74689566">src/mlpack/core/tree/hollow_ball_bound.hpp</a>:</p>
<pre style='color:#555'>&gt; @@ -35,6 +36,8 @@ class HollowBallBound
&gt;    math::RangeType&lt;ElemType&gt; radii;
&gt;    //! The center of the ball bound.
&gt;    arma::Col&lt;ElemType&gt; center;
&gt; +  //! The center of the hollow.
&gt; +  arma::Col&lt;ElemType&gt; hollowCenter;
</pre>
<p>The normal center is the center of the minimal bounding ball (i.e. the outer ball). And <code>hollowCenter</code> is the center of the hole (i.e. the inner ball).</p>

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