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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 link_reference(self, source, target):
 """Link the target to the source """

        target_uid = api.get_uid(target)
        # get the annotation storage key
        key = self.get_relationship_key(target)
        # get all backreferences from the source
        # N.B. only like this we get the persistent mapping!
        backrefs = get_backreferences(source, relationship=None)
        if key not in backrefs:
            backrefs[key] = PersistentList()
        if target_uid not in backrefs[key]:
            backrefs[key].append(target_uid)
        return True