A shadow that hangs over the debate on the Irish border and Brexit is the implied threat of renewed violence if anything happens to make the invisible border more obvious.
The Chief Constable of the Police Service of Northern Ireland has told Newsnight that he wouldn't want anything that looks like physical infrastructure at the border.
He fears it could be targeted by violent dissident republican groups - groups whose threat he defines as "severe".
The veteran documentary-maker and Northern Ireland observer Peter Taylor has this special report for Newsnight on the 'new' dissidents.
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