public static ReadOnlyList.SkipTakeEnumerable <List <TSource>, TSource> Skip <TSource>(this List <TSource> source, int count)
 => ReadOnlyList.Skip <List <TSource>, TSource>(source, count);
 public static ReadOnlyList.SkipTakeEnumerable <ImmutableArray <TSource>, TSource> Skip <TSource>(this ImmutableArray <TSource> source, int count)
 => ReadOnlyList.Skip <ImmutableArray <TSource>, TSource>(source, count);