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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 add(self, data, key=None):
 """Add field values to container. :param dict data: Serialized values for field :param str key: Optional key; if not provided, values will be added to `self.payload`. """

        sink = self.options[key] if key is not None else self.data
        for key, value in iteritems(data):
            sink.add(key, value)