namespace System.Linq
{
    public static class ParallelEnumerable
    {
        [Obsolete("The second data source of a binary operator must be of type System.Linq.ParallelQuery<T> rather than System.Collections.Generic.IEnumerable<T>. To fix this problem, use the AsParallel() extension method to convert the right data source to System.Linq.ParallelQuery<T>.")]
        public static ParallelQuery<TSource>! Union<TSource>(this ParallelQuery<TSource>! first, IEnumerable<TSource>! second);
    }
}