Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!uwm.edu!spool2.mu.edu!uunet!mcsun!ukc!acorn!osmith
From: osmith@acorn.co.uk (Owen Smith)
Newsgroups: comp.protocols.iso
Subject: Re: ISO/IEC 9596, Draft Addendum 2: ASN.1 syntax error?
Message-ID: <4638@acorn.co.uk>
Date: 10 Jan 91 21:31:20 GMT
References: <1991Jan9.155249.18149@cbnewsl.att.com>
Sender: osmith@acorn.co.uk
Distribution: comp
Organization: Acorn Computers Ltd, Cambridge, England
Lines: 12

I've come across this sort of thing before. The answer is that since the CHOICE cannot
possibly be IMPLICIT, it should promote to EXPLICIT automatically, and I know for a fact
that the PEPY compiler in ISODE 3.0 will not do this but gives an error instead. The Data
General ACY compiler will do the promotion. I admit that in this case it is a little silly
putting the IMPLICIT in and for the CHOICE to then promote to EXPLICIT. However, consider
the case when you are running with DEFINITIONS IMPLICIT TAGS set. If you now have a defined
type which is a CHOICE (as in the original example) any reference to it will be IMPLICIT
unless you stick an EXPLICIT keyword in, which is a pain hence the promotion feature.
The X.400 1988 spec has a few of these in it somewhere - I know because I had to stick
EXPLICIT in until we switched from PEPY to ACY.

Nuke 'em 'til they glow, then shoot 'em in the dark!