public static Parser Custom(Regex pattern, Func<Match, Node> map)
        {
            if (pattern == null) throw new ArgumentNullException("pattern");

            return text =>
            {
                var m = Consume(ref text, pattern);
                if (m == null) return ParseResult.Failure(text, pattern.ToString());

                return ParseResult.Success(map(m), text);
            };
        }

        public static Parser Custom(string uncompiledPattern, Func<Match, Node> map)
        {
            return Custom(new Regex(uncompiledPattern, _RegexOpts), map);
        }