message: Slave Plantation: +1 commerce from Slavery and Regulated Trade
diff: <iYield>1</iYield>
  </ImprovementYields>
  </ImprovementYieldChange>
  + <ImprovementYieldChange>
  + <ImprovementType>IMPROVEMENT_SLAVE_PLANTATION</ImprovementType>
  + <ImprovementYields>
  + <iYield>0</iYield>
  + <iYield>0</iYield>
  + <iYield>1</iYield>
  + </ImprovementYields>
  + </ImprovementYieldChange>
  <ImprovementYieldChange>
  <ImprovementType>IMPROVEMENT_MINE</ImprovementType>
  <ImprovementYields>
  <iYield>1</iYield>
  </ImprovementYields>
  </ImprovementYieldChange>
  + <ImprovementYieldChange>
  + <ImprovementType>IMPROVEMENT_SLAVE_PLANTATION</ImprovementType>
  + <ImprovementYields>
  + <iYield>0</iYield>
  + <iYield>0</iYield>
  + <iYield>1</iYield>
  + </ImprovementYields>
  + </ImprovementYieldChange>
  </ImprovementYieldChanges>
  <BuildingProductionModifiers>
  <BuildingProductionModifier>
  