ss_type</tt> keyword you see here is
equivalent to:</p>
<pre class="literal-block">
namespace boost { namespace python {

namespace tag { struct class_type; } // keyword tag type
template &lt;class T&gt;
struct class_type
  : parameter::template_keyword&lt;tag::class_type,T&gt;
{};

}}
</pre>
<!-- @example.prepend('#include <boost/parameter.hpp>') -->
<!-- @test('compile') -->
<p>It defines a keyword tag type named <tt class="docutils literal"><span class="pre">tag::class_type</span></tt> and a
<em>paramete