Bug in mapGroup: copy-to not allowed on topichead

The April 9 spec says this about the copy-to attribute (in DITA map
attributes):

"The @copy-to attribute also can be used to specify the name of a new chunk
when topics are being chunked; it also can be used to determine the name of
the stub topic that is generated from a <topicref> element that contains a
title but does not specify a target. In both of those cases, no duplicate
version of the topic is generated."

However, the declaration of topichead in the current mapGroup.mod does not
include copy-to. But other topicref specializations do, including topicset.

I think it should, otherwise you can never take advantage of this use of
@copy-to or specify an alternate name when using @chunk.

This looks like a simple oversight to me.

Cheers,

Eliot
-- 
Eliot Kimber
Senior Solutions Architect
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