Transaction-safe XML-CLOB Update

I wonder whether is it possible to update an XML document that is stored in
a RDBMS in an attribute of type CLOB in a transaction safe way? 

Consider the following example: Upon an SQL-Insert into relation R (occuring
in a transaction t) a trigger is fired (in the same transaction) that
updates an XML Document stored as an attribute of type CLOB in relation S.
When t
is aborted (i.e., a rollback takes place) I do not want the update that the
trigger performed on the XML document to be persistent.

Is it possible to write the trigger, i.e. the XML document update, in a
transaction safe way in general (independent of the employed DBMS)? However,
I
assume that it is only possible knowing the employed database. Since we use
Oracle, does anybody know how I can achieve this in Oracle9i?

Any help is highly appreciated.

Kind regards,
Martin Bernauer

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