/// <summary>
/// Inserts an item at the specified index, pushing the element at the index forward.
/// </summary>
/// <param name="index"> The index before which to insert the item. </param>
/// <param name="item"> The item to insert. </param>
/// <returns> </returns>
/// <exception cref="ArgumentOutOfRangeException">Thrown if the index doesn't exist.</exception>
public ImmList <T> Insert(int index, T item)
{
index.CheckIsBetween("index", -Root.Measure - 1, Root.Measure);
index = index < 0 ? Root.Measure + index + 1 : index;
if (index == Root.Measure)
{
return(AddLast(item));
}
if (index == 0)
{
return(AddFirst(item));
}
var newRoot = Root.Insert(index, item, Lineage.Mutable());
var ret = newRoot.Wrap();
ret[index].AssertEqual(item);
ret.Root.Measure.AssertEqual(Root.Measure + 1);
return(ret);
}