ABSTRACT
Think of the initial configuration of a data structure as version zero, and of every subsequent update operation as generating a new version of the data structure. Then a data structure is called persistent if it supports access to all versions and it is called ephemeral otherwise. The data structure is partially persistent if all versions can be accessed but only the newest version can be modified. The structure is fully persistent if every version can be both accessed and modified. The data structure is confluently persistent if it is fully persistent and has an update operation which combines more than one version. Let the version graph be a directed graph where each node corresponds to a version and there is an edge from node V1 to a node V2 if and only of V2 was created by an update operation to V1. For partially persistent data structure the version graph is a path; for fully persistent data structure the version graph is a tree; and for confluently persistent data structure the version graph is a directed acyclic graph (DAG).
