title: How to convert xt::sum expression result to an integer
body: <p>I am just starting out with xtensor and I am already stuck in a basic problem.</p>
  
  <p>I am using something like <code>xt::sum(xt::where(egoLaneLeftCount, 1, 0))</code> to sum a column and get a single integer value. I want to save this integer value into a variable but I get the following <code>compilation error</code>.</p>
  
  <pre><code>xt::xreducer&lt;xt::xreducer_functors&lt;std::plus&lt;long long int&gt;, xt::const_value&lt;long long int&gt;, std::plus&lt;long long int&gt; &gt;, xt::xfunction&lt;xt::detail::conditional_ternary, const xt::xarray_container&lt;xt::uvector&lt;bool, std::allocator&lt;bool&gt; &gt;, (xt::layout_type)1, xt::svector&lt;long unsigned int, 4, std::allocator&lt;long unsigned int&gt;, true&gt;, xt::xtensor_expression_tag&gt;&amp;, xt::xscalar&lt;int&gt;, xt::xscalar&lt;int&gt; &gt;, xt::svector&lt;long unsigned int, 4, std::allocator&lt;long unsigned int&gt;, true&gt;, xt::reducer_options&lt;long long int, std::tuple&lt;xt::evaluation_strategy::lazy_type&gt; &gt; &gt;
  </code></pre>
  
  <p>Any body has any idea how can I cast the result of this data type to an <code>int</code> ?</p>