I don't think you quite understand. A misordered binary search tree will fail lookups - (bst-find ...) will fail, even if that object is returned by (bst-elts ...). If you're going to create a general-purpose bst lib with "you can change the sort function without rebuilding the tree!" then you'd better make plenty damn sure there's a big warning saying "...but if you do (bst-find ...) might fail." Personally, I'd rebuild the tree anyway, because if the change is significant enough to change the sort function, it's big enough to completely destroy the meaning of the binary search tree.
I get that. When I think of using a bst I think of what News.YC needs: the top-ranked 100 or so stories out of 100,000, and no need for find or delete. (Though currently News.YC just truncates the list and sorts it, which would stop working at very high submission rates.)
A programming systems product takes about nine times as much effort as the component programs written separately for private use. I estimate that productizing imposes a factor of three; and that designing, integrating, and testing components into a coherent system imposes a factor of three; and that these cost components are essentially independent of each other.
