azy Default Computation</h3>
<p>When a default value is expensive to compute, it would be
preferable to avoid it until we're sure it's absolutely necessary.
<tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt> takes care of that problem for us, but
when using <span class="concept">ArgumentPack</span>s explicitly, we need a tool other than
<tt class="docutils literal">operator|</tt>:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(s1)
BOOST_PARAMETER_NAME(s2)
BOOST_PARAMETER_NAME(s3)

template &lt