public void NextItem()
{
var segment1 = new Segment("E", false);
var segment2 = new Segment("+", true);
var segment3 = new Segment("Number", true);
var production = new Production(segment1, ImmutableArray.Create(segment1, segment2, segment3));
var item0 = new ParseItem(production, 0);
var item1 = new ParseItem(production, 1);
var item2 = new ParseItem(production, 2);
var item3 = new ParseItem(production, 3);
Assert.That(item0.NextItem(), Is.EqualTo(item1));
Assert.That(item1.NextItem(), Is.EqualTo(item2));
Assert.That(item2.NextItem(), Is.EqualTo(item3));
Assert.That(item3.NextItem(), Is.EqualTo(null));
}