Many thanks to those who have replied to this with suggestions.

The solution

<rdg wit="E></rdg>

does indeed process successfully.  What I originally didn't like about
it (apart from a having a quasi-theological dislike of the idea of a tag
containing nothing) is purely practical: in a world where encoding is
likely to be endlessly interrupted by other pseudo-urgent demands, one
might eventually come back to this and wonder whether it was something
that never got finished ...

<rdg wit="E" type="silent"></rdg>

seems to get round most of this objection nicely.  As a matter of fact,
couldn't one also do

<rdg wit="E" type="silent"/>

?  It seems to work, and avoids using entities.

David Chadd
CURSUS Research Project
University of East Anglia
Norwich NR4 7TJ, UK
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