<p>I read through the X tree paper again.  I agree that the DescentType is not specified... the paper just says in Figure 7:</p>

<pre><code>follow = choose_subtree(obj);
</code></pre>

<p>And there is no explanation of what <code>choose_subtree()</code> is, so I think it's safe to assume that it's the typical R tree heuristic.  (Maybe it would be possible to use the R* heuristic too, but, I think sticking with the default is good for the X tree typedef.)</p>

<p>Ball trees aren't added to allkrann because of a tricky bug:<br>
<a href="https://github.com/mlpack/mlpack/issues/338" class="issue-link js-issue-link" data-url="https://github.com/mlpack/mlpack/issues/338" data-id="53048011" data-error-text="Failed to load issue title" data-permission-text="Issue title is private">#338</a></p>

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