n><span class="special">;</span>
<span class="comment">// initialize x</span>
<span class="identifier">explicit_rk4</span><span class="special">&lt;</span> <span class="identifier">state_type</span> <span class="special">&gt;</span> <span class="identifier">rk4</span><span class="special">;</span>
<span class="identifier">integrate_n_steps</span><span class="special">(</span> <span class="identifier">rk4</span> <span class="special">,</span> <span class="identifier">lorenz</span> <span class="special">,</sp