Simple Transaction Abort
 | For now, consider an explicit abort of a Xact.
 | No crash involved.
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| We want to “play back” the log in reverse order, UNDOing updates.
| Get lastLSN of Xact from Xact table.
| Can follow chain of log records backward via the prevLSN field.
| Before starting UNDO, write an Abort log record.
 | For recovering from crash during UNDO! | | | | |
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