xa'o was first proposed by Colin Fine: http://wiw.org/~jkominek/lojban/9301/msg00048.html
There] seems to be some confusion as to whether xa'o means "happening before the natural beginning" or "stopping before the natural end".
I vote for the former, "happening before the natural beginning", being the mirror image of "za'o" and also referring to a time span which the event does occur, which gives the latter to xa'o na.
In addition, I propose that bi'a be used for the standard word. xa'o sounds too close to za'o.
Well, that's why it was chosen such! I recall the original definition was that of stopping too soon. Beginning too soon or too late can be expressed as xa'o na or za'o na, respectively. I started going too late = mi za'o na klama (For too long I didn't go). Very elegant!
Either way you chose, the other meaning is "xa'o na". I like "happening before the natural beginning, because, like "za'o" it refers to a span when the event happens (and doesn't contain a negative), whereas "xa'o na" refers to a span when the event doesn't happen (and does contain a negative).
Then again, we could also create a new (experimental!) cmavo that indicates the "natural" time, as opposed to the actual time. With this, mi za'o klama could become something like mi ca'o (new cmavo) mo'u klama (I continue to go, but the natural stopping time of that going is now). This permits wackier combinations, where you can easily express "I went left, but it was natural that I'd gone up".
I can't seem to find the original proposal. If anyone knows where it is in the archives, could you please give us a link. Done