protected Upgrade bonusFor(int state) {
    switch (state) {
      case (DRILL_MELEE    ) : return Garrison.MELEE_TRAINING     ;
      case (DRILL_RANGED   ) : return Garrison.MARKSMAN_TRAINING  ;
      case (DRILL_ENDURANCE) : return Garrison.ENDURANCE_TRAINING ;
      case (DRILL_AID      ) : return Garrison.AID_TRAINING       ;
    }
    return null ;
  }