public void Scan_Null()
{
Assert.Throws <ArgumentNullException>(() => AsyncEnumerableEx.Scan(default(IAsyncEnumerable <int>), 3, (x, y) => x + y));
Assert.Throws <ArgumentNullException>(() => AsyncEnumerableEx.Scan(Return42, 3, default(Func <int, int, int>)));
Assert.Throws <ArgumentNullException>(() => AsyncEnumerableEx.Scan(default(IAsyncEnumerable <int>), (x, y) => x + y));
Assert.Throws <ArgumentNullException>(() => AsyncEnumerableEx.Scan(Return42, default(Func <int, int, int>)));
}
