public void cancelBehaviour(Behaviour b) {
    if (b == null) return ;
    if (! agenda.includes(b)) return ;
    while (agenda.size() > 0) {
      final Behaviour popped = popBehaviour() ;
      if (popped == b) break ;
    }
    if (agenda.includes(b)) I.complain("Duplicate behaviour!") ;
    actor.assignAction(null) ;
  }