def complement(chromosome, point1, point2):
    """
    Flip chromosome bits between point1 and point2.
    """
    new_chromosome = ""

    for i in range(len(chromosome)):
        if i >= point1 and i <= point2:
            if chromosome[i] == '0':
                new_chromosome += '1'
            else:
                new_chromosome += '0'
        else:
            new_chromosome += chromosome[i]

    return new_chromosome