How will the leadership candidates solve the Irish border question?

At the hacks’ hustings for the Tory leadership candidates, I asked the five who could be bothered to be held to account by your inky fingered servants a really boring question.

Would they accept the definition of a ‘hard border’ on the island of Ireland written into the December joint agreement between the UK and the EU, which underpins the backstop plan in the Withdrawal Agreement?

The reason this matters is that the joint agreement says there is a commitment to avoid ‘a hard border including any physical infrastructure or related checks and controls’, and the Withdrawal Agreement says ‘any future arrangements...