(rule (with-stdout-to x.gen (echo "")))
(rule (with-stdout-to y.gen (echo foobar)))

(rule
 (alias blah-non-existent-empty)
 (action (diff x-non-existent-empty x.gen)))

(rule
 (alias blah-non-existent-non-empty)
 (action (diff x-non-existent-non-empty y.gen)))
