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Bob Wyman wrote:
>[most of the useful and helpful info snipped]
Thanks. Very informative.
> SET and SEQUENCE in ASN.1 are statements about the permited
> lexical order of elements. In such cases that I've seen
> significance given to order, it has been done in
> specifications that accompany or incorporate the ASN.1
> definitions. However, the rules for Canonical and
> Distinguished encoding both require that the elements of SETs
> are, in fact, ordered according to their tag numbers. Thus,
> any semantic significance of order would be discarded if SETs
> were encoded with CER or DER.
So SET can only be used with CER and DER when order is semantically
insignificant. A case of performance trumping use cases, perhaps? (I'm
thinking of John Cowan's astute observation that when lexical order is
required order is usually not semantically significant, and when it is
not required, order is usually significant.)
> I'm not aware of any schema languages that allow the
> specification semantic signifance to order independent of
> lexical significance. Can you provide an example of one that
> does? I am curious to see how this concept is used and
> expressed.
Right. Now we've come full circle. I began by saying that no schema
language I'm aware of allows you to specify semantic significance
(independent of lexical), even though it would be good documentation and
useful for comparing XML documents.
Thanks again for taking the time to inform me and perhaps others on the
ins and outs of ASN.1 SET and SEQUENCE.
Bob Foster
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