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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 get_densities(self, spin=None):
 """ Returns the density of states for a particular spin. Args: spin: Spin Returns: Returns the density of states for a particular spin. If Spin is None, the sum of all spins is returned. """

        if self.densities is None:
            result = None
        elif spin is None:
            if Spin.down in self.densities:
                result = self.densities[Spin.up] + self.densities[Spin.down]
            else:
                result = self.densities[Spin.up]
        else:
            result = self.densities[spin]
        return result