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To customize the error messages that the Xerces or Saxon validation engines display for the <assert> and <assertion> elements, set the @message attribute on these elements.
  • For Xerces, the @message attribute has to belong to the http://xerces.apache.org namespace.
  • For Saxon, the @message attribute has to belong to the http://saxon.sourceforge.net/ namespace.
The value of the @message attribute is the error message displayed if the assertion fails.