Hi Ken,

Sorry to have abandoned this again. Your proposal is clearly workable in 
practice, but there are some issues with the implementation, of varying degrees 
of seriousness, that mean that it is not yet suitable for inclusion into CF. I 
agree that it seems like a minimally  intrusive set of changes, but some of the 
aspects you propose are a bit like the tip of the iceberg in complexity, like 
the ancillary variables containing ancillary variables issue.

The issue there is that the `ancillary_variables` attribute is not currently 
standardised on ancillary variables, so the linkage doesn't formally exist. If 
you made it meaningful when viewing a variable as an ancillary variable then 
this would need further additions to the conventions text _and_ the CF data 
model would need extending in as-yet-unknown ways. Before that route is 
explored we have to be sure that there is no way of representing the 
information you require with the existing machinery.

This could well be worse than it sounds! It is possible that minor changes 
could resolve these problems and make your proposal fit in with the CF view of 
data. I think that the key to progression might be to demonstrate clearly when 
any raised concerns are not in fact valid, or else be proactive in suggesting 
concrete alternative ideas, with examples, if none have been suggested.

I think I can devote some time to thinking about this in detail again towards 
the end of August, when I am back from leave, so I look forward to carrying on 
the discussion then.

All the best,
David

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