                <span class="comment">% (neigh_w is cleaned by searching the id</span>
                            <span class="comment">%  to avoid error when it is heterogeneous:</span>
                            <span class="comment">%  i.e. different wall types)</span>
                            p1.interacts(p1.interacts==int)   = [];
                            p1.neigh_w([p1.neigh_w.id]==w.id) = [];
                            p1.neigh_wid(p1.neigh_wid==w.id)  = [];

                            <spa