DFDL 1.0 has the idea of a floating element which when declared in a sequence is allowed to appear anywhere in the sequence. See if that gets you what you need.  Spec section 14.4.

Regards

Steve Hanson
Architect, IBM Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848



From:        "Garriss Jr., James P." <jgarriss@mitre.org>
To:        "dfdl-wg@ogf.org" <dfdl-wg@ogf.org>,
Date:        01/04/2013 14:55
Subject:        [DFDL-WG] IMF Comments
Sent by:        dfdl-wg-bounces@ogf.org

---

Comments can appear anywhere in the value of an email header.  The initiator is an open paren, and the terminator a closed paren.  They are meant only for human readers, thus they can be safely ignored by processors.  These are equivalent:
 
     MIME-Version: 1.0
     MIME-Version: 1.0 (produced by MetaSend Vx.x)
     MIME-Version: (produced by MetaSend Vx.x) 1.0
     MIME-Version: 1.(produced by MetaSend Vx.x)0
 
I don’t see comments used in too many headers, but they are used frequently in the Received header:
 
Received: from e06smtp13.uk.ibm.com (e06smtp13.uk.ibm.com [195.75.94.109])            by
smtpksrv1.mitre.org (Postfix) with ESMTP id 655301F069B           for
<jgarriss@mitre.org>; Tue, 12 Mar 2013 08:02:00 -0400 (EDT)
 
How would you handle this in DFDL?  Does it have a mechanism for saying, “This element can appear anywhere and any number of times inside of this other element?”--
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