code: def specstring(self):
          
          if self.subgroup is None:
              variable = self.variable
          else:
              variable = f'{self.subgroup}.{self.variable}'
          if self.series:
              variable = f'{variable}.series'
          return variable
docs: The string corresponding to the current values of `subgroup`,
          `state`, and `variable`.
  
          >>> from hydpy.core.itemtools import ExchangeSpecification
          >>> spec = ExchangeSpecification('hland_v1', 'fluxes.qt')
          >>> spec.specstring
          'fluxes.qt'
          >>> spec.series = True
          >>> spec.specstring
          'fluxes.qt.series'
          >>> spec.subgroup = None
          >>> spec.specstring
          'qt.series'