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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 xindex(self):
 """Positions of the data on the x-axis :type: `~astropy.units.Quantity` array """

        try:
            return self._xindex
        except AttributeError:
            self._xindex = Index.define(self.x0, self.dx, self.shape[0])
            return self._xindex