Placing underground belt between existing connection should mark disconnected input belt as terminal

[sparr]

If you place two underground belts so that they are connected, then place another one between them, the new one will connect to one of the first two and disconnect the other of the first two. If the disconnected one is an input, it needs to be marked as terminal.

This would be easier with a pre_built event. Failing that, it will require tracing backwards from the new neighbour to find any potential previous connections.