Called by TransformationSimulationRule and did not work with the
following use case (where child_BL1):

  BL(trade_phase=TP1, successor=trade_state/TS1):
    * child_BL1(predecessor=trade_state/TS1,
                trade_phase=TP1/InventoryAccounting)
    * child_BL3(predecessor=trade_state/TS1,
                trade_phase=Transforming)