code: def append_sfixed64(self, value):
          
          sign = (value & 0x8000000000000000) and -1 or 0
          if value >> 64 != sign:
              raise errors.EncodeError('SFixed64 out of range: %d' % value)
          self._stream.append_little_endian64(value & 0xffffffffffffffff)
docs: Appends a signed 64-bit integer to our buffer, in little-endian
          byte-order.