ation</span>

        <span class="comment">% Contact parameters</span>
        unload_stiff <span class="string">double</span> <span class="string">=</span> <span class="string">double.empty</span>;   <span class="comment">% unloading stiffness coefficient</span>
        unload_param <span class="string">double</span> <span class="string">=</span> <span class="string">double.empty</span>;   <span class="comment">% variable unload stiffness coefficient parameter</span>
        residue      <span class="stri