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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 unlink_version(self, source, target):
 """Unlink the current version of the target from the source """

        if not hasattr(source, REFERENCE_VERSIONS):
            return
        target_uid = api.get_uid(target)
        if target_uid in source.reference_versions[target_uid]:
            # delete the version
            del source.reference_versions[target_uid]
            # persist changes that occured referenced versions
            source._p_changed = 1
        else:
            logger.warn("No version link found on '{}' -> '{}'"
                        .format(repr(source), repr(target)))