 recommendations</a>, the
      target class is often required to supply the assignment counterpart
      operator of the concept's "main operator." For example, the
      <code>addable</code> template requires <code>operator+=(T
      const&amp;)</code> and in turn supplies <code>operator+(T const&amp;, T
      const&amp;)</code>.</li>
    </ol>

    <h3>Use of <i><a name="concepts_note">concepts</a></i></h3>

    <p>The discussed concepts are not necessarily the standard library's
    concepts (CopyConstr