tr can be the invalid iterator after last element or before first element, their index will be regarded as last element's index + 1 and -1 respectively. </p>
          </div>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idp51063120"></a>Return Value</h4>
              </div>
            </div>
          </div>
          <p>The index difference. </p>
        </div>
        <d