<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'>> +#define MLPACK_CORE_DATA_MAP_POLICIES_MISSING_POLICY_HPP
> +
> +#include <mlpack/core.hpp>
> +#include <unordered_map>
> +#include <boost/bimap.hpp>
> +#include <mlpack/core/data/map_policies/datatype.hpp>
> +#include <limits>
> +
> +
> +using namespace std;
> +
> +namespace mlpack {
> +namespace data {
> +
> +/**
> + * 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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