once-only: execute arbitrary code but only the first time it's encountered. https://github.com/akkartik/readwarp/blob/master/utils.arc#1
init: like assign, but only if name isn't already bound. For arbitrary variables besides tables. https://github.com/akkartik/readwarp/blob/master/utils.arc#L...
inittab: probably same as your proposal, just a different name. Even uses `or=` under the hood. https://github.com/akkartik/readwarp/blob/master/utils.arc#L...