M-label definition

[Alexander Mayrhofer]

> won't making M-label a subset of U-label destroy an important 
> property  
> of U-label as defined in IDNA2008?
> 
> Under IDNA2008 there is a one-to-one mapping of every U-label to A- 
> label and vice-versa, without any mapping.
> 
> M-label needs mapping to become a U-label and is thus a superset, I  
> believe.

I fully agree. I think that "M-Labels" would definitely be a superset of
"U-Labels", and the relation might be as follows:

- Any valid U-Label can also be considered an M-Label (because of that
superset property, "null" mapping..)
- many M-Labels may map to a single U-Label (because the mapping might
loose information)
- therefore, the mapping M-Label -> U-Label is not reversible

Also:

- A string that contains codepoints for which no mapping is defined (eg.
Unassigned) is just a string, not an "M-Label". (or, "any valid M-Label
must be successfully mapped to a U-Label"?)

Does that sound roughly right?

Alex