I just requested to be put on the mailing list and will reach out to those folks. I'm not sure if they realized the potential of this notation. (Or maybe they have long known this--I don't want to assume anything).

It appears Egil Möller had 90% of the details correct years ago! This is really well explained and clear. I am not surprised to see this come from the Scheme world. TN/ETNs and LISP are quite closely related.

I think the distinction between TN and the ETNs is important (and not having the additional S-expr construct in the base). Also, a minor technical detail or two in the implementation that matters in terms of anti-fragility.

Thanks again, really super helpful. The number of notations and languages I've carefully looked at is over 300 (I'll post a full list soon), but I've long suspected that others have been super close or already found TN and ETNs.