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?”