invariants and can also <span class="emphasis"><em>subcontract</em></span>
      inheriting and extending contracts from base classes (see <a href="../../example/features/introduction_public.cpp" target="_top"><code class="literal">introduction_public.cpp</code></a>
      and <a class="link" href="boost_contract/tutorial.html#boost_contract.tutorial.public_function_overrides__subcontracting_" title="Public Function Overrides (Subcontracting)">Public
      Function Overrides</a>): <a href="#ftn.boost_contrac