<p>In <a href="https://github.com/mlpack/mlpack/pull/716#discussion_r69716514">src/mlpack/methods/pca/pca_impl.hpp</a>:</p>
<pre style='color:#555'>&gt;  #include &lt;mlpack/core.hpp&gt;
&gt; +#include &quot;pca.hpp&quot;
&gt;  
&gt;  using namespace std;
</pre>
<p>It's used for:</p>

<p><code><br>
Log::Fatal &lt;&lt; "PCA::Apply(): varRetained (" &lt;&lt; varRetained &lt;&lt; ") must be "<br>
        &lt;&lt; "greater than or equal to 0." &lt;&lt; endl;<br>
</code></p>

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