Defining SelectMany method's signature for Parser type

Parsers.cs

var p1 = startSriper.ToParser();
var p2 = endSriper.ToParser();

var composite = p1.Compose(p2);

var result2 = composite(content);

var parsers = from a in p1
              from b in p2
              select b;

var result3 = parsers(content);


public static Parser<TSource, TResult> Bind<TSource, TResultTemp, TResult>(this Parser<TSource, TResultTemp> p, Func<TResultTemp, ParseResult<TResult>> func)
        {
            return input =>
            {
                var r1 = p(input);
                var ar1 = r1 as Success<TResultTemp>;

                if (ar1 == null)
                {
                    return new Error<TResult>(((Error<TResultTemp>)r1).Message);
                }

                return func(ar1.Parsed);
            };
        }

        public static Parser<TSource, TResult> SelectMany<TSource, TResultTemp, TCollection, TResult>(this Parser<TSource, TResultTemp> source,
                        Func<TResultTemp, Parser<TResultTemp, TCollection>> collectionSelector,
                        Func<TResultTemp, TCollection, TResult> resultSelector)
        {
            return source.Bind(a => collectionSelector(a).Bind(b => resultSelector(a, b).ToParseResult())(a));
        }