docs: 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