  </h7><p>
            Returns the index corrosponding to the first character of <code class="computeroutput"><span class="special">{</span><span class="identifier">begin</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">pos</span><span class="special">,</span> <span class="identifier">end</span><span class="special">())</span></code>
            that is not equal to <code class="computeroutput"><span class="identifier">ch</span></code>
            if it exists, a