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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 read_varint64(self):
 """Reads a varint from the stream, interprets this varint as a signed, 64-bit integer, and returns the integer. """

        i = self.read_var_uint64()
        if i > wire_format.INT64_MAX:
            i -= (1 << 64)
        return i