an class="keyword">typename</span> DerivIn<span class="special">,</span> 
         <span class="keyword">typename</span> StateOut<span class="special">&gt;</span> 
  <span class="keyword">void</span> <a name="idp20672064-bb"></a><span class="identifier">do_step</span><span class="special">(</span><span class="identifier">System</span> system<span class="special">,</span> <span class="keyword">const</span> <span class="identifier">StateIn</span> <span class="special">&amp;</span> in<span class="special">,</s