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

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