Lookup for reserved LDH labels

[Simon Josefsson]

John C Klensin <klensin at jck.com> writes:

> Remember that IDNA2008 explicitly, and after long discussion,
> forbade lookup of XN-- strings that were not A-labels in spite
> of understanding that there were native character strings out
> there and registered that did not conform to the U-label rules.

Can you point to the spec text here?  I see in section 5.3 text that
could be relevant, but it is marked with MAY, not MUST or even SHOULD.

If I read through section 5 of RFC 5891, considering lookup of the
string "xn--foo-" (which is not a valid A-label), I don't see find any
text that says it shouldn't be looked up.

/Simon