/// <summary>
/// Returns a data-producer that yeilds the values from the sequence, or which yields the default
/// value for the Type if no data is produced.
/// </summary>
/// <param name="source">The source data-producer.</param>
public static IDataProducer <TSource> DefaultIfEmpty <TSource>(this IDataProducer <TSource> source)
{
return(source.DefaultIfEmpty(default(TSource)));
}