|I mentioned this in a comment on "New Features" a while back but there was no reply ... but I'm sure it's important!|
arc> (ssexpand 'a+~b)
The problem is that expand-compose is triggered by ~ occurring intra-symbol, but expand-compose only expands initial ~.
(andf a ~b)
See http://arclanguage.org/item?id=9960 for more details, including what I think may be a fix