Seq.cs

using System;
using System.Collections;

public abstract class IntSeq : IEnumerable
{
    public abstract int Size { get; set; }

    public abstract void clear();
    public abstract void unshift(int item);
    public abstract int shift();
    public abstract void push(int item);
    public abstract int pop();

    public abstract IntSeqEnumerator GetEnumerator();

    IEnumerator IEnumerable.GetEnumerator()
    {
        return (IEnumerator) GetEnumerator();
    }
}

public abstract class IntSeqEnumerator : IEnumerator
{
    object IEnumerator.Current { get => Current; }
    public abstract int Current { get; }
    public abstract bool MoveNext();
    public abstract void Reset();
}