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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 specstring(self):
 """The string corresponding to the current values of `subgroup`, `state`, and `variable`. 'fluxes.qt' 'fluxes.qt.series' 'qt.series' """

        if self.subgroup is None:
            variable = self.variable
        else:
            variable = f'{self.subgroup}.{self.variable}'
        if self.series:
            variable = f'{variable}.series'
        return variable