public Ghost matchGhost(Element e, Model m) {
    final Vec3D p = e.sprite().position ;
    final Section section = world.sections.sectionAt((int) p.x, (int) p.y) ;
    List <Ghost> SG = ghosts.get(section) ;
    Ghost match = null ;
    if (SG != null) for (Ghost g : SG) {
      if (g.tracked == e && g.sprite.model() == m) { match = g ; break ; }
    }
    return match ;
  }