/// <summary>
/// Returns a future to a single value from a data-source or the default value if no values
/// are encountered. An exception
/// is thrown if multiple values are encountered.
/// </summary>
/// <param name="source">The source data-producer.</param>
/// <exception cref="InvalidOperationException">Multiple terms are encountered.</exception>
public static IFuture <TSource> SingleOrDefault <TSource>(this IDataProducer <TSource> source)
{
return(source.SingleOrDefault(x => true));
}