<p>In <a href="https://github.com/mlpack/mlpack/pull/686#discussion_r66710185">src/mlpack/methods/ne/population.hpp</a>:</p>
<pre style='color:#555'>> @@ -0,0 +1,89 @@
> +/**
> + * @file population.hpp
> + * @author Bang Liu
> + *
> + * Definition of Population class.
> + */
> +#ifndef MLPACK_METHODS_NE_POPULATION_HPP
> +#define MLPACK_METHODS_NE_POPULATION_HPP
> +
> +#include <cstddef>
> +
</pre>
<p>It should be already included if you include <code>core.hpp</code>.</p>
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