code: def unlink_version(self, source, target):
          
          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)))
docs: Unlink the current version of the target from the source