I don't remember when I first released FSet. I did write it before Clojure showed up (2004-ish). Anyway, by "now" I just meant that FSet was a relatively recent arrival in the history of CL, not that it was necessarily newer than Clojure.

Agreed on both counts about map default values. I've been wanting to change how they work, but the change I have in mind would be incompatible. Here's what I'm thinking. Instead of every map (and seq) having a default, which defaults to NIL, we would distinguish maps with defaults from those without. Doing LOOKUP on a defaultless map at a key it has no mapping for would signal a condition (similarly for an out-of-bounds lookup on a defaultless seq). Then the defaultless versions could be used more conveniently with UNION (and COMPOSE, another place where I've tripped over this problem).

I was thinking that creating a map without supplying a default should give you a defaultless map, and that's how I wish I had done it from the beginning, but making it work that way now would break a lot of code. I could release it as a new major version (and call it "fset-2" in Quicklisp so people don't pull it down accidentally). Or, I could leave the existing constructor behavior unchanged and just add a new defaultless map (and seq) constructor. I think this is considerably less elegant, but it wouldn't break existing code. What do you think?