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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 append_sint64(self, value):
 """Appends a 64-bit integer to our buffer, zigzag-encoded and then varint-encoded. """

        zigzag_value = wire_format.zig_zag_encode(value)
        self._stream.append_var_uint64(zigzag_value)