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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 AddDatastore(self, urn):
 """Adds a datastore URN as a source. Args: urn: an RDF URN value of the datastore. Returns: True if the datastore is not an already existing source. """

    if urn not in self._datastores:
      self._datastores.add(urn)
      return True
    return False