@davidhassell commented on this pull request.
> +
+A data variable defines its domain via its own attributes, but a
+domain variable provides the description of a domain in the absence of
+any data values. It is of arbitrary type since it contains no data. It
+acts as a container for the attributes that define the domain. The
+purpose of a domain variable is to provide domain information to
+applications that have no need of data values at the domain's
+locations, thus removing any ambiguity when retrieving a domain from a
+dataset. Ancillary variables and cell methods are not part of the
+domain, because they are only defined in relation to data values.
+
+The domain variable supports the same attributes as are allowed on a
+data variable for describing a domain, with exactly the same meanings
+and syntaxes, as described in <<attribute-appendix>>. If an attribute
+is needed by a particular data variable to describe its domain, then
+that attribute would also be needed by the equivalent domain variable.
I appreciate your concern, here. I feel that the approach is guaranteed to work
for as long as CF still insists that a data variable encodes is own, implicit
domain, which is to say for the foreseeable future.
My one point of concern, however, is for a scalar data variable that innocently
has the `dimensions` attribute - how can we tell if it is a data variable or a
domain variable?
Perhaps using the `dimensions` attribute as the identifying feature could be
replaced by the existing `cf_role` attribute. I think that this could be a bit
safer:
```
char domain ;
domain:cf_role = "domain" ;
domain:dimensions = "x y" ;
domain:coordinates = "lat lon" ;
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