t possibly influenced by either the <code class="computeroutput"><span class="identifier">lower</span><span class="special">[]</span></code>
                      or <code class="computeroutput"><span class="identifier">upper</span><span class="special">[]</span></code>
                      directives.
                    </p>
                    <p>
                      The return value defines the outcome of the whole generator.
                      If it is <code class="computeroutput"><span class="ke