swap

// swap two elements of list:
// based on answer here https://stackoverflow.com/questions/552731/c-good-best-implementation-of-swap-method

    List<Boid> Swap(List<Boid> list, int indexA, int indexB)
    {
        Boid tmp = list[indexA];
        list[indexA] = list[indexB];
        list[indexB] = tmp;
        return list;
    }