+** METHOD: sqlite3_snapshot
+**
+** ^The [sqlite3_snapshot_open(D,S,P)] interface either starts a new read
+** transaction or upgrades an existing one for schema S of
+** [database connection] D such that the read transaction refers to
+** historical [snapshot] P, rather than the most recent change to the
+** database. ^The [sqlite3_snapshot_open()] interface returns SQLITE_OK
+** on success or an appropriate [error code] if it fails.
+**
+** ^In order to succeed, the database connection must not be in
+** [autocommit mode] when [sqlite3_snapshot_open(D,S,P)] is called. If there
+** is already a read transaction open on schema S, then the database handle
+** must have no active statements (SELECT statements that have been passed
+** to sqlite3_step() but not sqlite3_reset() or sqlite3_finalize()).
+** SQLITE_ERROR is returned if either of these conditions is violated, or
+** if schema S does not exist, or if the snapshot object is invalid.
+**
+** ^A call to sqlite3_snapshot_open() will fail to open if the specified
+** snapshot has been overwritten by a [checkpoint]. In this case
+** SQLITE_ERROR_SNAPSHOT is returned.
+**
+** If there is already a read transaction open when this function is
+** invoked, then the same read transaction remains open (on the same
+** database snapshot) if SQLITE_ERROR, SQLITE_BUSY or SQLITE_ERROR_SNAPSHOT
+** is returned. If another error code - for example SQLITE_PROTOCOL or an
+** SQLITE_IOERR error code - is returned, then the final state of the
+** read transaction is undefined. If SQLITE_OK is returned, then the
+** read transaction is now open on database snapshot P.