BOOST_DECLARE_IS_CALLABLE macro extends the functionality provided by
        BOOST_DECLARE_HAS_MEMBER and allows to declare a trait which would then let
        introspect the existence of a named class member function <span class="emphasis"><em>callable
        with the supplied signature</em></span>.
      </p>
<p>
        For example, the following declarations introduce <code class="computeroutput"><span class="identifier">local</span><span class="special">::</span><span class="identifier">can_call_fun