<p>In <a href="https://github.com/mlpack/mlpack/pull/757#discussion_r73979743">src/mlpack/methods/ann/layer/concat_layer.hpp</a>:</p>
<pre style='color:#555'>&gt; +  {
&gt; +    ForwardTail(layers, output);
&gt; +  }
&gt; +
&gt; +  template&lt;size_t I = 0, typename DataType, typename... Tp&gt;
&gt; +  typename std::enable_if&lt;I == sizeof...(Tp), void&gt;::type
&gt; +  ForwardTail(std::tuple&lt;Tp...&gt;&amp; layers, DataType&amp; output)
&gt; +  {
&gt; +    /* Nothing to do. */
&gt; +  }
&gt; +
&gt; +  template&lt;size_t I = 0, typename DataType, typename... Tp&gt;
&gt; +  typename std::enable_if&lt;I &lt; sizeof...(Tp), void&gt;::type
&gt; +  ForwardTail(std::tuple&lt;Tp...&gt;&amp; layers, DataType&amp; output)
&gt; +  {
&gt; +    output = arma::join_slices(output, std::get&lt;I&gt;(layers).OutputParameter());
</pre>
<p>The problem I see here is, that it only works if we use cubes as output type. We could use template specializations to distinguish between arma::mat and arma::cube what do you think?</p>

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