Prevent decrement underflow when rendering messages with long prefixes on small terms.
?: =(who our.bol) aud
(~(del in aud) [who %inbox])
==
- =. wyd (sub wyd (lent pef))
+ =. wyd (sub wyd (min (div wyd 2) (lent pef)))
=/ txt (tuba (trip msg.sep))
|- ^- (list tape)
?~ txt ~
