Intuitively, this sweep process works like commitmentTx in lightning, where the client always have an exit transaction available in case the counterparty refuses to respond.
In Boomerang, before a Fragment signs on the SweepTx, the new Fragment Judge proposes a Refund Transaction on the previous state of the reserve. Once all the new Fragment signatories sign on the refund transaction, the Sweep signing begins on the unlocked Fragment address.
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