[ietf-dkim] Attempted text for x=

[Paul Hoffman]

This looks pretty good! One major stumbling block, though.

At 4:44 PM +0100 4/19/06, Stephen Farrell wrote:
>Proposed:
>. . .
>        Verifiers SHOULD support checking of x= values.

 From RFC 2119, which DKIM normative references:

3. SHOULD   This word, or the adjective "RECOMMENDED", mean that there
    may exist valid reasons in particular circumstances to ignore a
    particular item, but the full implications must be understood and
    carefully weighed before choosing a different course.
. . .
    Imperatives of the type defined in this memo must be used with care
    and sparingly.  In particular, they MUST only be used where it is
    actually required for interoperation or to limit behavior which has
    potential for causing harm (e.g., limiting retransmisssions)  For
    example, they must not be used to try to impose a particular method
    on implementors where the method is not required for
    interoperability.

What is the interoperability or harm-limiting purpose of verifiers 
checking x= values? If there is none, the sentence above needs to be 
a MAY.