You can use IF/THEN/ELSE to provide custom results, but you should still write plural relevance that returns the real and complete results. Once you have the complete results, then you can filter using a whose statement so you can check for specific results. Then you can assert with a TRUE/FALSE that those results exist if that is what you want to do. Only then should you use an IF/THEN/ELSE to provide different values rather than TRUE/FALSE, but, you should keep in mind that IF/THEN/ELSE is going to be less efficient than using the plural relevance alone.