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From: Markku.Savela@tel.vtt.fi (Markku Savela)
Newsgroups: comp.protocols.iso
Subject: Re: ASN.1 defn. is a grammar?
Message-ID: <5370@hemuli.tik.vtt.fi>
Date: 1 Feb 91 07:34:48 GMT
References: <1991Jan31.165316.616@shl.com>
Sender: news@hemuli.tik.vtt.fi
Reply-To: savela@tel.vtt.fi (Markku Savela)
Organization: Technical Research Centre of Finland
Lines: 25
In-reply-to: guest@shl.com's message of 31 Jan 91 16:53:16 GMT

In article <1991Jan31.165316.616@shl.com> guest@shl.com (Guest csh account (should be sh)) writes:

> I have an ASN.1 specification from a nameless client which falls (I
> think) into the LL(k) where k > 1 category.  The gist of this
> definition is:
>    Choice1 ::= CHOICE { [0] type1, [1] type2, [2] type3 }
>    Choice2 ::= CHOICE { [1] type4 }
>    Maintype ::= SEQUENCE {
>        first  Choice1 OPTIONAL,
>        second Choice2
>    }

   That defition is not valid ASN.1 The tags of the first and
second must be distinct:

   25:      Maintype ::= SEQUENCE {
   26:          first  Choice1 OPTIONAL,
   27:          second Choice2
 ** ERROR on line   27 : Non-distinct tag: [Non-distinct tag: [1]

(I think that the syntax defined by ASN.1 is *always* LL(1) or LL(0)). 

--
Markku Savela (savela@tel.vtt.fi), Technical Research Centre of Finland
Telecommunications Laboratory, Otakaari 7 B, SF-02150 ESPOO, Finland