code: def append_varint64(self, value):
          
          if not wire_format.INT64_MIN <= value <= wire_format.INT64_MAX:
              raise errors.EncodeError('Value out of range: %d' % value)
          if value < 0:
              value += (1 << 64)
          self.append_var_uint64(value)
docs: Appends a signed 64-bit integer to the internal buffer,
          encoded as a varint.