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