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Beside this sequence of immutable revisions, the filesystem can contain an arbitrary number of transactions, which are mutable snapshots that have no successors and whose predecessors (base revisions) are immutable. The only way to create a new revision is to merge the tree mutations described by a transaction into the HEAD tree and append the result to the revision sequence, thereby creating a new HEAD (in other words, rebase the transaction to HEAD and make it immutable).
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