Thanks, Eric, that was a brilliant reply—and you set me off on the right track.
I’ve realized what I’ve done—all I had to do was type the magic words
sudo apt-add-repository ppa:swi-prolog/devel
and install from there!
?- Dict=pl_en{kot:cat, pies:dog}, X = Dict.get(panda, unknown).
Dict = pl_en{kot:cat, pies:dog},
X = unknown. % as desired!
Q: Did I miss something in the web documentation that informs a predicate/function is in the dev version only?
Extra: If you’ve ended up here but you’re stuck on a pre-8.3 version, there’s an implementation of get-with-default in the other thread here.