A-label definition

[Frank Ellermann]

Sorry, I got <x-label> wrong.  There is no "third hyphen"
if the U-label contains no LDH:

wrong:  x-label = "xn--"  *<l-d-h> "-"  1*<let-dig> 
fixed:  x-label = "xn--" [*<l-d-h> "-"] 1*<let-dig>

The punycode detail is not more interesting in the fixed
form, just  x-label = "xn--" *<l-d-h> <let-dig>  will do.

Any valid A-label is still an x-label, and any x-label
is still a top-label.  

xn--cocacola and xn--foo are <x-labels>, they no need no
third hyphen to become more obscure.  And if they are no
valid A-label they are still ldh-labels.  

Maybe the <x-label> syntax could help in the draft, it's
clearer than talking about "potential A-labels", if the
term A-label implies valid.

 Frank