public void Expand_Null()
{
Assert.Throws <ArgumentNullException>(() => AsyncEnumerableEx.Expand(default(IAsyncEnumerable <int>), x => default(IAsyncEnumerable <int>)));
Assert.Throws <ArgumentNullException>(() => AsyncEnumerableEx.Expand(Return42, default(Func <int, IAsyncEnumerable <int> >)));
}