<p>In <a href="https://github.com/mlpack/mlpack/pull/664#discussion_r67910746">src/mlpack/core/tree/rectangle_tree/hilbert_r_tree_split.hpp</a>:</p>
<pre style='color:#555'>&gt; + * Defintion of the HilbertRTreeSplit class, a class that splits the nodes of an R
&gt; + * tree, starting at a leaf node and moving upwards if necessary.
&gt; + */
&gt; +#ifndef MLPACK_CORE_TREE_RECTANGLE_TREE_HILBERT_R_TREE_SPLIT_HPP
&gt; +#define MLPACK_CORE_TREE_RECTANGLE_TREE_HILBERT_R_TREE_SPLIT_HPP
&gt; +
&gt; +#include &lt;mlpack/core.hpp&gt;
&gt; +
&gt; +namespace mlpack {
&gt; +namespace tree /** Trees and tree-building procedures. */ {
&gt; +
&gt; +/**
&gt; + * The order of the splitting policy. The Hilbert R tree splits a node
&gt; + * on overflow, turnung splitOrder node to (splitOrder+1) nodes.
&gt; + */
&gt; +constexpr int splitOrder = 2;
</pre>
<p>I'll make it a template parameter. I didn't test the values those are not equal to 2. Maybe some other values will be better.</p>

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