ChainedList

public class ChainedList<T>
{
  internal readonly RecycleList<Link> _list = [];
  internal readonly record struct Link(T Value, int Next);
}

public sealed class Chain<T>
{
  public ChainedList<T> List { get; }
  public T Current
  {
    get => List[_head].Value;
    set => List[_head] = new(value, List[_head].Next);
  }

  internal Chain(ChainedList<T> list, int index)
  {
    List = list;
    _prev = -1;
    _head = index;
  }

  public bool MoveNext()
  {
    int next = List[_head].Next;
    _prev = _head;
    return (_head = next) > 0;
  }

  public void AddBefore(T value)
  {
    int index = List.NextIndex();
    List[_prev] = new(List[_prev].Value, index);
    List[index] = new(value, _head);
  }

  public void AddAfter(T value)
  {
    int index = NextIndex();
    List[index] = new(value, List[_head].Next);
    List[_head] = new(List[_head].Value, index);
  }

  public bool Remove()
  {
    int next = List[_head].Next;
    List[_prev] = new(List[_prev].Value, next);
    List.Recycle(_head, default);
    return (_head = next) > 0;
  }

  private int _prev;
  private int _head;
}