[fitsbits] INHERIT and Hierarchical Grouping

[Malcolm J. Currie]

Just glancing through the proposal, I noticed that it continues to
misuse EXTNAME, although understandable given an omission from the
generalized extensions paper.  (Sorry Bill if I sound like a broken
record on this point.)  GROUPING is a type; it describes the function or
semantics of the HDU or component in a data structure, it is not a named
instance.

Don added these EXT* keywords specifically with Starlink hierarchical
data in mind.  It's my fault for not picking the omission of an EXTTYPE
keyword from the extension paper before publication.  It only came
blindly obvious once I tried to convert a Starlink NDF dataset into FITS
and back to the hierarchical format.  Once I'd added an EXTTYPE to
record the component data type, to supplement the path within the
hierarchy stored in EXTNAME, it was possible to convert to FITS and back
to NDF recovering the original structure.

"Generalized extensions and blocking factors for FITS", Section 9 does
suggest using the component paths "to establish easy to understand
relationships between different extensions and even between different
extensions in different FITS files."

The overall concept looks good and goes beyond the single file.
However, I should re-read it more carefully.

I do prefer the VOTable way of grouping as explained by Doug, through
familiarity with that perspective in our own packages.

Malcolm Currie
Rutherford Appleton Laboratory