[fitsbits] Abuse of EXTEND keyword

[LC's NoSpam Newsreading account]

On Thu, 23 Aug 2007, Preben Grosbol wrote:

> I prefer such flags (GROUPS, BLOCKED, EXTEND) as they, by nature,
> just indicated an additional convention. 

I share this preference, provided the convention is useful and actually 
used ... anyhow this does not imply the presence of the flag be 
mandatory.

On Wed, 22 Aug 2007, Tim Pearson wrote:

> The Grosbøl et al. paper (1988 A&A) said:
> 
> "Note that the presence of EXTEND=T in a primary FITS header ...
> indicates that the file _may_ have extensions records and that any
> special records will conform to the rules below." The important part    
> of this statement is the last bit, "any special records will conform 
> to the rules below"

Thanks Tim for reminding us of the second part of the statement.

However I wonder about the actual situation. I consider three cases 
for the EXTEND keyword, and four things it may signal (the possibility 
to have extensions, the presence of actual extensions, the presence of 
special records, their conformance to the standard). The answer to each 
thing can be YES NO or MAYBE.

But what does "conformance of special records to the standard" mean ?
Am I correct in assuming that in 1988 parliance "conforming extensions" 
were one possible kind of "generalized special records" while in 2007 
parliance "conforming extensions" are no longer "special records" ... 
or at least the (deprecated) "special records" are all other possible 
kinds of "generalized special records" (mutually exclusive with 
"conforming extensions") ?

I note that EXTEND=F was never *explicitly* defined

I try to make this truth table ...

           may have ext's  HAS ext's  uses spec rec  they conform
---------------------------------------------------------------------
no EXTEND  yes             maybe      maybe          maybe         
EXTEND=T   yes             maybe      maybe          yes if present
EXTEND=F   ??              ??         maybe          no if present

I can't figure out if the "??" shall be NO or "maybe". But definitely 
the "maybe" in the first two lines are "maybe", so they are equivalent

... which seems to indicate that a mandatory EXTEND=T is not needed 
(which apparently is just the change in the wording of FITS 3.0 ... but 
no practical change in usage ?)

Lucio Chiappetti

-- 
----------------------------------------------------------------------
nospam at mi.iasf.cnr.it is a newsreading account used by more persons to
avoid unwanted spam. Any mail returning to this address will be rejected.
Users can disclose their e-mail address in the article if they wish so.