Persistent Zipper data-structure for the win in C#

ImZipper.cs

using System;

public class Test
{
	public static void Main()
	{
		var x = ImZipper<int>.Empty.Pre(1).Pre(2);
		Console.WriteLine(x);
		Console.WriteLine(x.Focus);
	}
}

public static class Ext
{
	public static ImList<T> Reverse<T>(this ImList<T> l)
	{
		var r = ImList<T>.Empty;
		for (var x = l; !x.IsEmpty; x = x.Tail)
			r = r.Push(x.Head);
		return r;
	}
}

public class ImZipper<T>
{
	public static readonly ImZipper<T> Empty = new ImZipper<T>(ImList<T>.Empty, ImList<T>.Empty);
	public bool IsEmpty => this == Empty;
	public readonly ImList<T> LRev, R;
	public ImList<T> L => _l ?? (_l = LRev.Reverse());
	private ImList<T> _l;
	public T Focus => IsEmpty ? default (T) : R.IsEmpty ? LRev.Head : R.Head;
	public ImZipper<T> Post(T x) => new ImZipper<T>(LRev, R.Push(x));
	public ImZipper<T> Pre(T x) => new ImZipper<T>(LRev.Push(x), R);
	public ImZipper(ImList<T> lrev, ImList<T> r)
	{
		LRev = lrev;
		R = r;
	}

	public override string ToString() => L + "," + R;
}

public class ImList<T>
{
	public static readonly ImList<T> Empty = new ImList<T>(default (T), null);
	public bool IsEmpty => this == Empty;
	public readonly T Head;
	public readonly ImList<T> Tail;
	public ImList<T> Push(T h) => new ImList<T>(h, this);
	private ImList(T h, ImList<T> t)
	{
		Head = h;
		Tail = t;
	}

	public override string ToString() => IsEmpty ? "()" : Head + "::" + Tail;
}