gistfile1.xq

http:set-status(400);

(: with the content-type "application/json" the json xml is automatically serialized to JSON :)
http:set-content-type("application/json");

(: this is the xml representation of JSON which makes it easy to generate it, and work with it within sausalito :)
<json type="object">
<pair name="status" type="number">400</pair>
<pair name="statusmessage" type="string">Bad Request</pair>
<pair name="message" type="string">Please, enter a search term</pair>
</json>;