5.6 Terminating Nodes are Recursively Evaluated

It is important to understand that terminating nodes are recursively evaluated until they resolve to a <message> tag. This means, e.g., that an engine can return an <engine> tag that references another engine that in turn returns another <engine> tag, etc., until ultimately the last engine returns a <message> tag. Similar possibilities apply to the <query> tag. The system internally resolves the <rss> tag to a <message> tag.