Returns maximum negative value or None.

def max_neg(self):
        '''Returns maximum negative value or None.'''
        if self.__len__() == 0:
            return ArgumentError('empty set has no maximum negative value.')
        if self.contains(0):
            return None
        negative = [interval for interval in self.intervals
                    if interval.right < 0]
        if len(negative) == 0:
            return None
        return numpy.max(list(map(lambda i: i.right, negative)))