  Transactional memory allows the user to declare sequences of instructions as
speculative \emph{transactions} that can either \emph{commit} or \emph{abort}.
If a transaction commits, it appears to be executed sequentially, so that the
committed transactions constitute a correct sequential execution. If a
transaction aborts, none of its instructions can affect other transactions.
  The popular criterion of \emph{opacity} requires that the views of aborted
transactions must also be consistent with the global sequential order
constituted by committed ones. This is believed to be important, since
inconsistencies observed by an aborted transaction may cause a fatal
irrecoverable error or waste of the system in an infinite loop. Intuitively, an
opaque implementation must ensure that no intermediate view a transaction
obtains before it commits or aborts can be affected by a transaction that has
not started committing yet, so called \emph{deferred-update} semantics.
  In this paper, we intend to grasp this intuition formally. We propose a
variant of opacity that explicitly requires the sequential order to respect the
deferred-update semantics. We show that our criterion is a safety property,
i.e., it is prefix- and limit-closed. Unlike opacity, our property also ensures
that a serialization of a history implies serializations of its prefixes.
Finally, we show that our property is equivalent to opacity if we assume that
no two transactions commit identical values on the same variable, and present a
counter-example for scenarios when the "unique-write" assumption does not hold.
