private static IEnumerable <ListOperation <T> > Enumerate([NotNull] IListInsertOperation <T> insert)
        {
            var index = insert.Index;

            foreach (var item in insert.Items)
            {
                yield return(ListOperation <T> .OnAdd(item, index ++));
            }
        }
 private static ItemOrEnumerable <ListOperation <T> > OnInsert([NotNull] IListInsertOperation <T> insert) =>
 new ItemOrEnumerable <ListOperation <T> >(Enumerate(insert));