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Unbonding is triggered when the PoS contract is invoked (again in the normal way) with the signature of an existing validator (this signature could be on the hash of the parent of the block after which they want to be unbonded) along with a return channel. On the return channel, an unforgable name is sent, which the now ex-validator can listen on. After the post-unbonding stake holding time has elapsed, a purse containing the ex-validaor's stake is sent on that name, thus allowing the validator to recover their bond. Note that no rewards are distributed at this time, only the exact original stake (minus any slashed amount) is returned. Rewards can be withdrawn form from the shard's local PoS contract after unbonding has occurred (see below). The unbonding is successful so long as it does not exceed the unbonding rate specified for the region.
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