private static float reservedForCollection(
    Batch <Behaviour> doing, Service goodType
  ) {
    float sum = 0 ;
    for (Behaviour b : doing) {
      if (b instanceof Delivery) for (Item i : ((Delivery) b).items) {
        if (i.type == goodType) sum += i.amount ;
      }
    }
    return sum ;
  }