</span> <span class="identifier">DerivIn</span> <span class="special">&amp;</span> dxdt<span class="special">,</span> <span class="identifier">time_type</span> t<span class="special">,</span> 
          <span class="identifier">time_type</span> dt<span class="special">)</span><span class="special">;</span></pre>The method performs one step. Additionally to the other method the derivative of x is also passed to this method. It is supposed to be used in the following way: <pre class="programlisting"> sys( x ,