I have no idea what "k" "a" or "d" is.
"a" means yes to all, "d" means no to all, "k" means undo the previous choice. I don't know why those letters were chosen, I just got this from reading the code...
I asked on the IRC channel - I think it's:
@joeneeman you beat me to it :-) And I was also wrong about k
I think I'm guilty for that choice.
These were chosen to match the commands in darcs. Also, at the time, pijul was versionned using darcs.