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A subtransaction indicates usage of the :meth:`.Session.begin` method in conjunction with
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the ``subtransactions=True`` flag. This produces a non-transactional, delimiting construct that
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allows nesting of calls to :meth:`~.Session.begin` and :meth:`~.Session.commit`.
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both independently of any external code that starts a transaction,
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as well as within a block that has already demarcated a transaction.