The vanquished challengers move further to the left each year, and Jeremy Corbyn's victories over them get bigger.
In 2015, the most left-wing candidate other than Corbyn was Andy Burnham, and even Liz Kendall was on the ballot.
Corbyn won big.
In 2016, Corbyn won even bigger against Owen Smith, who was well to the left of Burnham.
In 2017, will he win even bigger again against Clive Lewis, who is well to the left of Smith?