public static async Task FoldAsyncImplicit_NullValue_Throws(Func <int, int, Task <int> > folder)
{
var actual = await Record.ExceptionAsync(() => CollectionExtensions.FoldAsync(null, folder))
.ConfigureAwait(false);
var ane = Assert.IsType <ArgumentNullException>(actual);
Assert.Contains("collection", ane.Message, Ordinal);
}