           The common domain is the strongest domain in set <span class="emphasis"><em>P</em></span>, with the
                following caveats.
              </li>
<li class="listitem">
                Let <span class="emphasis"><em>U</em></span> be the union of all sets
                <span class="emphasis"><em>S<sub>N</sub></em></span>. If the result is 
                <code class="computeroutput"><a class="link" href="default_domain.html" title="Struct default_domain">proto::default_domain</a></code>