def _simple_breed(best, num, mutation_rate, random_f):
    """Returns num copies of parent with mutation_rate changes"""
    result = []
    score, parent = best
    for child_number in range(num):
        if random_f() <= mutation_rate:
            child = parent.mutate()
            result.append(child)
        else:
            result.append(parent)
    return result
