
          Python container to behave.
        </div>
      </li>
      <li>
        Provide default reference semantics for container element indexing
        (<tt>__getitem__</tt>) such that <tt>c[i]</tt> can be mutable.
        Require: 
        <div>
          <pre>
    val = c[i]
    c[i].m()
    val == c[i]
          </pre>
        </div>where <tt>m</tt> is a non-const (mutating) member function
        (method).
      </li>
      <li>
        Return safe references from <tt>__getitem__</tt> such that