int main(void) {

  cons_cell *points, *pl;

  point p1 = {1,2}, p2 = {3,4}, p3 = {5,6}, p_new = {11,12};

  points = cons(&p1,
                cons(&p2,
                     cons(&p3,
                          NULL)));


  insert_after(&p_new, tail(points));  

  pl = points;    
  while (pl != NULL) {
    prnt_point(*((point*)(head(pl))));
    pl = tail(pl);
  }

  printf("\n\n");

  delete_after(points);

  pl = points;    
  while (pl != NULL) {
    prnt_point(*((point*)(head(pl))));
    pl = tail(pl);
  }

  return 0;
}