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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 _merge_map(key, values, partial):
 """A map function used in merge phase. Stores (key, values) into KeyValues proto and yields its serialization. Args: key: values key. values: values themselves. partial: True if more values for this key will follow. False otherwise. Yields: The proto. """

  proto = kv_pb.KeyValues()
  proto.set_key(key)
  proto.value_list().extend(values)
  yield proto.Encode()