Returns the number of elements in the skiplist with a score
        between min and max.

def count(self, minval, maxval, include_min=True, include_max=True):
        '''Returns the number of elements in the skiplist with a score
        between min and max.
        '''
        rank1 = self.rank(minval)
        if rank1 < 0:
            rank1 = -rank1 - 1
        elif not include_min:
            rank1 += 1
        rank2 = self.rank(maxval)
        if rank2 < 0:
            rank2 = -rank2 - 1
        elif include_max:
            rank2 += 1
        return max(rank2 - rank1, 0)