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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def error(self):
 """Returns the error for this barrier and all work items, if any."""

        # Copy the error from any failed item to be the error for the whole
        # barrier. The first error seen "wins". Also handles the case where
        # the WorkItems passed into the barrier have already completed and
        # been marked with errors.
        for item in self:
            if isinstance(item, WorkItem) and item.error:
                return item.error
        return None