public static TSource Single <TSource>(this IReadOnlyList <TSource> source, Func <TSource, bool> predicate)
        {
            switch (source)
            {
            case TSource[] array:
                return(ArrayExtensions.Single <TSource>(array, predicate));

            default:
                return(ReadOnlyList.Single <IReadOnlyList <TSource>, IEnumerator <TSource>, TSource>(source, predicate));
            }
        }
        public static TSource Single <TSource>(this IEnumerable <TSource> source, Func <TSource, bool> predicate)
        {
            switch (source)
            {
            case TSource[] array:
                return(ArrayExtensions.Single <TSource>(array, predicate));

            default:
                return(SingleDowncasted <IEnumerable <TSource>, TSource> .Single(source));
            }
        }