> And, I didn't go with #ifMissingAdd: because it confuses the return value with the linguistic semantics -- e.g., if it returns true, does that mean it was "absent" or that it was added?  forcing me to look at the method implementation again anyway.

I think I confused this with a different one.  ifMissingAdd:
actually seems fine in terms of the return value semantics..  "true"
would mean, "yes", it was missing and added.

>From there, saying we want to stay with the "Absent" terminology in the
API seems reasonable.

+1 for ifAbsentAdd:.