ion introduced
<a class="reference internal" href="#binding-intro">earlier</a>:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(index)

template &lt;class ArgumentPack&gt;
typename parameter::value_type&lt;ArgumentPack, tag::index, int&gt;::type
twice_index(ArgumentPack const&amp; args)
{
    return 2 * args[_index|42];
}

int six = twice_index(_index = 3);
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <boost/type_traits/remove_reference.hpp>
#include <cassert>

namespace par