 is
        in this case a simple function calculating <span class="emphasis"><em>f(x)</em></span>. The
        parameter signature of this function is crucial: the integration methods
        will always call them in the form <code class="computeroutput"><span class="identifier">f</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span>
        <span class="identifier">dxdt</span><span class="special">,</span>
        <span class="identifier">t</span><span class="