docs: def min_pos(self):
          '''Returns minimal positive value or None.'''
          if self.__len__() == 0:
              return ArgumentError('empty set has no minimum positive value.')
          if self.contains(0):
              return None
          positive = [interval for interval in self.intervals
                      if interval.left > 0]
          if len(positive) == 0:
              return None
          return numpy.min(list(map(lambda i: i.left, positive)))