No subject

*might* be a useful term, because it is as far as ABNF can get
us without actually trying to decode a (wannabe) A-label.

And 'anything with an "--", not preceded by "xn"' would match=20
"abc--f", but not "axn--f".  Both are not interesting for IDN.

Any 'not preceded by "xn"' rule is also difficult in ABNF; we
would need LX (letters excluding "x") and LN to talk about it
in this syntax (in addition to simple L, D, H definitions).

 Frank