/// <summary>
/// Matches the specified <paramref name="parser"/> when the specified <paramref name="notParser"/> does not match.
/// </summary>
/// <typeparam name="TSource">The type of the source elements.</typeparam>
/// <typeparam name="TNotResult">The type of the elements that are generated by the <paramref name="notParser"/>.</typeparam>
/// <typeparam name="TResult">The type of the elements that are generated from parsing the source elements.</typeparam>
/// <param name="parser">The parser to match when the specified <paramref name="notParser"/> does not match.</param>
/// <param name="notParser">The parser that when it matches will cause the matches from the specified
/// <paramref name="parser"/> to be ignored.</param>
/// <returns>A parser that matches the specified <paramref name="parser"/> when the specified <paramref name="notParser"/>
/// does not match.</returns>
public static IObservableParser <TSource, TResult> Not <TSource, TNotResult, TResult>(
this IObservableParser <TSource, TResult> parser,
IObservableParser <TSource, TNotResult> notParser)
{
Contract.Requires(parser != null);
Contract.Requires(notParser != null);
Contract.Ensures(Contract.Result <IObservableParser <TSource, TResult> >() != null);
return(notParser.None(default(TNotResult)).SelectMany(_ => parser, (_, result) => result));
}