How do we solve the Brexit border problem?
Ireland finds itself on the front line of that endlessly long-running, still-unresolved battle between harder and softer variants of Brexit.
The whole of Ireland has a population one tenth the size of the UK; Northern Ireland's population is smaller than West Yorkshire - yet it is here that the argument is being fought.
An argument that involves three borders: Britain and Northern Ireland, Northern Ireland and the Republic, and the Republic and Britain.
Evan Davis looks at how we solve the Brexit border problem.
(Image: Northern Ireland border, credit: BBC)