Fuzzy set gradable membership score
        See http://code.google.com/p/python-colormath/wiki/ColorDifferences

def membership_score(self, element):
        """ Fuzzy set gradable membership score
        See http://code.google.com/p/python-colormath/wiki/ColorDifferences
        """
        other = self.coerce(element)
        if self.value and other.value:
            return 1 - (self.value.delta_e(other.value, mode='cmc', pl=1, pc=1) / 200.0)
        else:
            return 0.0