I'm not sure if this is a supported scenario, but the following deadlocks in Clojure 1.3:
Cause: As I understand it, the problem is that ys' fill takes place on an agent thread, so when it calls xs' drain, the (send-off agt fill) does not immediately trigger xs' fill, but is instead put on the nested list to be performed when ys' agent returns. Unfortunately, ys' fill will eventually block trying to take from xs, and so it never returns and the pending send-offs are never sent.
Approach: Use (release-pending-sends) in seque's drain function to avoid the deadlock when a seque is fed into another seque.
Screened by: Alex Miller
Windows 7; JVM 1.6; Clojure 1.3 beta 1