[spfbis] RFC7208 dual-cidr-length clarification

[Jan Schaumann <[e-mail]>]

Hello,

Reading RFC7208 leaves me wondering whether the
correct syntax when specifying both an ip4-cidr-length
and an ip6-cidr-length requires two slashes to
separate them:

a                = "a"      [ ":" domain-spec ] [ dual-cidr-length ]
ip4-cidr-length  = "/" ("0" / %x31-39 0*1DIGIT) ; value range 0-32
ip6-cidr-length  = "/" ("0" / %x31-39 0*2DIGIT) ; value range 0-128
dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]

This reads to me that the correct syntax would need to
be:

a:example.com/24//64
             +--++--+
             || |||
             || ||+---> "64" (ip6-cidr-length value range 0-128)
             || ||
             || |+----> "/" from 'ip6-cidr-length' definition
             || |
             || +-----> "/" from 'dual-cidr-length' definition
             ||
             |+-------> "24" (ip4-cidr-length value range 0-32)
             |
             +--------> "/" from 'ip4-cidr-length' definition

I suspect that the "/" in 'dual-cidr-length' is
intended to allow an empty ip4-cidr-length, so that
it'd be possible to say

a:example.com//64

to convey that no ip4-cidr-length is specified (and
would then be equivalent to "a:example.com/32/64"),
but I think as written above, specifying both an
ip4-cidr-length and an ip6-cidr-length would require
dual-slashes as shown above, rather than the more
intuitive

a:example.com/24/64

Perhaps the intended grammar was

dual-cidr-length = ( "/" / [ ip4-cidr-length ] ) [ ip6-cidr-length ]

?

Thanks for any clarification here,
-Jan