docs: def min_abs(self):
          '''Returns minimum absolute value.'''
          if self.__len__() == 0:
              return ArgumentError('empty set has no minimum absolute value.')
          if self.contains(0):
              return 0
          return numpy.min([numpy.abs(val)
                            for val in [self.max_neg(), self.min_pos()]
                            if val is not None])