Protection of hyphen

If it's worth protecting punctuation characters at all, then I think
hyphen comes right at the top of the list of characters that it is
useful to protect (probably tying with plus in importance).

Indeed, the draft give the example of owner-listname (1.3) which isn't
actually protected by the current proposal.

One simple approach would be to leave underscore unprotected instead,
use xn__ as the ACE prefix and to use _ as the delimiter in
bootstring.  Underscore is one of the few characters that would be
safe to use in this way, since it has longstanding use in e-mail
addresses of the form initials_surname.

(I confess however that I don't understand the explanation of the
infix solution to this problem mentioned in the draft; perhaps an
example would be helpful?)

Of course, others may object underscore is also used in structured
localparts, but it's undoubtedly less common than hyphen.

	-roy