I liked Boris's summing up on this today. This is how it went:

Imagine the scenario: Corbyn wins the election and goes to Brussels with his alternative Brexit plan. Remember he is "neutral" with regards the plan. He meets Barnier who politely asks to see the details of his new plan. Having scanned through it he turns to Corbyn:

Barnier: Monsieur Corbyn do you fully support this plan?

Corbyn: Monsieur Barnier I cannot comment on the plan as I am remaining neutral.

Barnier: Monsieur Corbyn: do Monsieur McDonald and Madame Abbott and the rest of your cabinet support this plan?

Corbyn: Er, actually no, they are promoting remain.

The outcome is pretty predictable, laughable if it were not so serious.