span></code>
        macro fails.
      </p></div>
<div id="ftn.boost_contract.introduction.f2" class="footnote"><p><a href="#boost_contract.introduction.f2" class="para"><sup class="para">[4] </sup></a>
        The <code class="computeroutput"><span class="identifier">pushable</span></code> base class is
        used in this example just to show subcontracting, it is somewhat arbitrary
        and it will likely not appear in real code.
      </p></div>
<div id="ftn.boost_contract.introduction.f3" class="f