Thanks, Randal. I think what you suggested doesn't allow for something like

<open>stuff <open>stuff <close><close>

although it would allow

<open><open>stuff <close><close>

Or maybe I misunderstood. At any rate, it's food for thought and I will ponder on it (and report back if you were right and I was wrong...).

Have a nice trip!

thomas. Note for clarity: this was first reply to Randal (before his update), though it appears here lower in the string