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Returns true if the entry location on the conveyor is blocked. Blocked means that an entity has control of the cell. A blockage causes non-accumulating conveyors to stop. For accumulating conveyors, items can continue moving until they reach the blockage and cannot move forward through the blockage. The entry cell remains blocked until released. The entity does not have to be on the conveyor to block the entry. The entity can release the blockage by exiting the conveyor (before getting on) or after riding on the conveyor. A blockage at an entry location is automatically released the instant that an item riding on a conveyor is completely on the conveyor. That is, when the item's rear most cell first occupies the entry cell.