He makes some really excellent points. This is what he had to say about it:
"Government and Opposition have both accepted that the Government can't declare its position to Parliament as the negotiations progress. Theresa May can't tell Parliament that she has three twos and a couple jacks and then try to bluff in Brussels that she is holding a Royal Flush.
"But how can Parliament possibly vote on the outcome of a ...two-year negotiating process? It would be a Yes-Yes vote. Parliament can't reject Brexit without provoking a constitutional crisis and it cannot reject the terms of Brexit when there are no other terms on offer. The Government can't ask for more time to negotiate because the time period is set in the Treaty. It can't be varied without a Treaty amendment.
"Could the Government negotiate two alternative paths and let Parliament choose between them? That's a problem too. Our partners would not see this as negotiating in good faith. Furthermore, either the Government or our partners could seek to stack the deck by offering a false choice between a favoured option and something such as "invade Poland". (H/T "Yes, Prime Minister" for this old civil service technique). It is also likely that neither option would be as good as could be secured by focusing on a single set of terms."