Plasma white paper (page 19):
"The fraud proofs ensure that a coalition of participants are not able to create fraudulent blocks without getting penalized. In the event a fraudulent block is detected and proven on the root blockchain (or parent Plasma chains), the invalid block is rolled back."
How can a block in an independent chain (Plasma chain) be reverted? If I'm a Plasma operator, how can Ethereum main chain make me revert a block, how is that enforced?
And what if there are valid blocks already built on top of the invalid one?