<p>In <a href="https://github.com/mlpack/mlpack/pull/694#discussion_r69801775">src/mlpack/core/data/map_policies/missing_policy.hpp</a>:</p>
<pre style='color:#555'>> +#include <limits>
> +
> +
> +using namespace std;
> +
> +namespace mlpack {
> +namespace data {
> +
> +/**
> + * Same as increment map policy, but does not change type of features.
> + */
> +class MissingPolicy
> +{
> + public:
> + // typedef of mapped_type
> + using mapped_type = double;
</pre>
<p>Also, I'm not sure I get the intention to use an alias here. Why not just use <code>double</code>?</p>
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