I got this idea from XKCD's Hofstadter comic; what's the best way to create a conditional loop in (any) Lisp dialect that executes a function until it returns NIL at which time it collects the returned values into a list.

For those who haven't seen the joke, it's goes that Douglas Hofstadter's “eight-word” autobiography consists of only six words: “I'm So Meta, Even This Acronym” containing continuation of the joke: (some odd meta-paraprosdokian?) “Is Meta” — the joke being that the autobiography is actually “I'm So Meta, Even This Acronym Is Meta”. But why not go deeper?

Assume the acronymizing function META that creates an acronym from a string and splits it into words, returns NIL if the string contains but one word:

(meta "I'm So Meta, Even This Acronym") ? "Is Meta"
(meta (meta "I'm So Meta, Even This Acronym")) ? "Im"
(meta (meta (meta "I'm So Meta, Even This Acronym"))) ? NIL

(meta "GNU is Not UNIX") ? "GNU"
(meta (meta "GNU is Not UNIX")) ? NIL

Now I'm looking for how to implement a function so that:

(so-function #'meta "I'm So Meta, Even This Acronym") 
? ("I'm So Meta, Even This Acronym" "Is Meta" "Im")
(so-function #'meta "GNU is Not Unix")
? ("GNU is Not Unix" "GNU")

What's the best way of doing this?