public static void LstAppendRange(int[] xs, IEnumerable <int> elem)
{
var fl = new Lst <int>(xs);
var origLength = fl.Count;
var fl2 = fl.AppendRange(elem);
var list = new List <int>(xs ?? new int[0]);
list.AddRange(elem);
Assert.Equal(list, fl2);
Assert.Equal(origLength, fl.Count);
Assert.Equal(origLength + elem.Count(), fl2.Count);
}