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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 AddClientLabels(self, client_id, owner, labels):
 """Attaches a user label to a client."""

    if client_id not in self.metadatas:
      raise db.UnknownClientError(client_id)

    labelset = self.labels.setdefault(client_id, {}).setdefault(owner, set())
    for l in labels:
      labelset.add(utils.SmartUnicode(l))