<p>In <a href="https://github.com/mlpack/mlpack/pull/694#discussion_r69909352">src/mlpack/core/data/map_policies/missing_policy.hpp</a>:</p>
<pre style='color:#555'>&gt; +#define MLPACK_CORE_DATA_MAP_POLICIES_MISSING_POLICY_HPP
&gt; +
&gt; +#include &lt;mlpack/core.hpp&gt;
&gt; +#include &lt;unordered_map&gt;
&gt; +#include &lt;boost/bimap.hpp&gt;
&gt; +#include &lt;mlpack/core/data/map_policies/datatype.hpp&gt;
&gt; +#include &lt;limits&gt;
&gt; +
&gt; +
&gt; +using namespace std;
&gt; +
&gt; +namespace mlpack {
&gt; +namespace data {
&gt; +
&gt; +/**
&gt; + * Same as increment map policy, but does not change type of features.
</pre>
<p>Could you detail what the class does a little more?  I think that, based on reading it, <code>MissingPolicy</code> takes a set of tokens and maps those tokens to <code>NaN</code>---but it also maps any other tokens that can't be converted to numbers to <code>NaN</code>.  Is that correct?  If so, we should document it better, I think.</p>

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