[Exim] Router Preconditions

It seems that the default behavior of a router - when a file is missing
- is to defer. I'm converting my Exim 3 to Exim 4 and the require_files
command no longer works the way it did and the exists command is very ugly.

What I really want - if I could have my way - is a command that tells
the router to skip to the next router of files are missing. Then I
wouldn't need either the require_files or exists command. Does such a
command already exist? If not - could it be added?