private void byAttribute()
        {
            TokenQueue cq = new TokenQueue(tq.ChompBalanced('[', ']')); // content queue
            string key = cq.ConsumeToAny("=", "!=", "^=", "$=", "*=", "~="); // eq, not, start, end, contain, match, (no val)

            if (key.Length == 0)
                throw Failure.EmptyString("key");

            cq.ConsumeWhitespace();

            if (cq.IsEmpty) {
                if (key.StartsWith("^", StringComparison.Ordinal))
                    evals.Add(new Evaluator.AttributeStarting(key.Substring(1)));
                else
                    evals.Add(new Evaluator.Attribute(key));
            } else {
                if (cq.MatchChomp("="))
                    evals.Add(new Evaluator.AttributeWithValue(key, cq.Remainder()));

                else if (cq.MatchChomp("!="))
                    evals.Add(new Evaluator.AttributeWithValueNot(key, cq.Remainder()));

                else if (cq.MatchChomp("^="))
                    evals.Add(new Evaluator.AttributeWithValueStarting(key, cq.Remainder()));

                else if (cq.MatchChomp("$="))
                    evals.Add(new Evaluator.AttributeWithValueEnding(key, cq.Remainder()));

                else if (cq.MatchChomp("*="))
                    evals.Add(new Evaluator.AttributeWithValueContaining(key, cq.Remainder()));

                else if (cq.MatchChomp("~="))
                    evals.Add(new Evaluator.AttributeWithValueMatching(key, new Regex(cq.Remainder())));
                else
                    throw DomFailure.CannotParseAttributeQuery(query, cq.Remainder());
            }
        }