I am writing a WordPress plugin that does string processing whenever 'the_author' filter event is fired. Obviously, there are more plugins that might respond to this event. I know that you can set the priority of your plugin but for example the Disqus plugin is called before my plugin even when I set my priority to 0. (I don't know if negative values are accepted). The Codex says about the $priority: "functions with the same priority are executed in the order in which they were added to the action".
So my question is, how can I make sure that my plugin is added first to the action? or alternatively, can you use negative values for priority?