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In Factorio 1.1.9, Smart Belt Dragging was added to the game engine, allowing automatic placement of underground belts when dragging regular belt through obstacles.
To support this feature, belts need the 'related_underground_belt' property defined: https://wiki.factorio.com/Prototype/TransportBelt#related_underground_belt
This message has been manually sent, based on automatic detection of your mod defining (either through explicit definition or use of deepcopy) its own belt prototypes.
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