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To make changes to the voter set safe it is required that the majority of the competing voter sets commit the voter changes. In this design the competing voter sets are the current voter set and new voter set. Since this design only allows one voter change at a time the majority of the new configuration always overlaps (intercepts) the majority of the old configuration. This is done by the leader committing the current epoch when it becomes leader and committing single voter changes with the new voter set before accepting another voter change.
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