
      <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 subsequent

        adds and delete