private String itemReference(Item item) {
        Integer reference = itemReferences.get(item);
        return reference != null ? reference.toString() : "Unknown item: '"  + System.identityHashCode(item) + "'";
    }
Creates a detailed representation of a query tree.

@author Tony Vaagenes
container-search/src/main/java/com/yahoo/prelude/query/textualrepresentation/TextualQueryRepresentation.java