code: def system(self):
          
          if self._base == 2:
              return "NIST"
          elif self._base == 10:
              return "SI"
          else:
              # I don't expect to ever encounter this logic branch, but
              # hey, it's better to have extra test coverage than
              # insufficient test coverage.
              raise ValueError("Instances mathematical base is an unsupported value: %s" % (
                  str(self._base)))
docs: The system of units used to measure an instance