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Subject: Re: [ubl] Extension element children

Good to hear from you John. I was probably lax in my phrasing
- I meant the extension would be * displaced * (not replaced) as last
element when the document is extended in any minor version. In other
words, as Mike wrote, the extension element may be last in the document
now but, though keeping its position in the sequence, perhaps not stay
as the last element in a minor version which extends the document.

Glad to hear things in ASN.1 follow so closely the same principles for
extension as do those in XSD. This helps the confidence of both being
interchangable even when there have been extensions, either through
minor versioning (with backwards compatibility, by definition) or through
use of the extension element or, hopefully, even with both.

In response to Mike (Mike's explanation much clearer than mine) I
have some sympathy for the extension element being at the start of
the document so its relative position stays the same (and therefore
simplifies a rule to say where it should appear). That's if it makes no
other difference.

All the best


On 11/04/06, John Larmouth <j.larmouth@salford.ac.uk> wrote:
> I am not sure whether you are talking ASN.1 or something else.
> But for ASN.1, the extension marker can appear anywhere in a SEQUENCE etc, but
> if it is in the middle, the syntax is (eg):
> Fred ::= SEQUENCE {
> first   First,
> second  Second,
> ...,
> ...,
> third   Third,
> fourth  Fourth}
> The next version puts additional elements between the two extension markers.
> Please note (you probably already know) that if you want backwards
> compatability, the extension markers have to be present in "version 1",  and you
> cannot move them in "version 2".
> John L
> Stephen Green wrote:
> > Is there any reason why the extension * has * to be at the end of
> > the document? The reason I ask is that there may, if so, be an issue
> > with further extending the document with minor versions within UBL,
> > mightn't there? Such extensions would * have * to be at the end of
> > the document - for backwards compatibility. They would therefore
> > replace the extension element as the last element in the document.
> > I'm hoping the extension element is only placed at the end of the
> > document for aesthetic or common practise reasons.
> >
> > All the best
> >
> > Steve
> >
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> >
> --
>     Prof John Larmouth
>     Larmouth T&PDS Ltd
>     (Training and Protocol Development Services Ltd)
>     1 Blueberry Road
>     Bowdon                               j.larmouth@salford.ac.uk
>     Cheshire WA14 3LS
>     England
>     Tel: +44 161 928 1605               Fax: +44 161 928 8069

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