[fitsbits] Start of the CONTINUE keyword Public Comment Period

Perhaps the discussion from 1993 and 1994 could be included verbatim,
or linked by thread off the web page? I'd hate to hold a debate on a
topic that was already batted around a dozen years ago. Is the
registry page the entire text of the proposed convention? Like Tom
said, you need to cover the obvious corner cases, at least by
verbiage like "it is undefined what string is conveyed if a card
ending is '&' is not followed by CONTINUE".

1) Couldn't the reordering question be dealt with via appending &NNN
instead of &? That could resolve some of the corner cases, too, and
you wouldn't need to reserve multiple CONT_NNN keywords. We should
be leery of implicitly requiring that all FITS headers be now
considered strictly ordered, when there is no practical way to
enforce this. In particular, this has the side effect of making the
interpolation of any new keyword questionable other than immediately
before END.

2) What is the reason for not strictly requiring that such a
continuation string end with ampersand-quote? What advantage is
conveyed by permitting semantically null trailing whitespace that
merely confuses the handling? Surely it should be incumbent on the
author of such keywords to trim meaningless characters, not
generations of readers yet unborn to dodge around them.

3) One suspects a number of conventions will depend on valueless
keywords. The standard (2.1b) is rather convoluted on how this works:

5.1.2.2 Value Indicator (bytes 9–10)
If this field contains the ASCII characters “=”, it indicates the
presence of a value field
associated with the keyword, unless it is a commentary keyword as
defined in §5.4.2.4.
If the value indicator is not present or if it is a commentary
keyword then columns 9–80
may contain any ASCII text.

Which basically just acts to preserve the legality of COMMENT,
HISTORY and blank keywords to have an equals sign in column 9.

4) In general, the nature of a convention versus a standard is that
others are perfectly within their rights to violate a convention for
either good or not-so-good reasons. The behavior here seems simple,
but that's where the risk lies. Nobody would be unsurprised that
deleting the INHERIT keyword turns inheritance off - it's almost a
feature. Delete the CHECKSUM keyword and you simply remove an
assertion about the contents of the file. One can even productively
use DATASUM and CHECKSUM independently. (I'll have to check if we
made that legal.) On the other end of the spectrum, conventions like
tiled image compression and foreign file encapsulation involve very
specific FITS usage - a file must adhere to the standard and
additionally must adhere to the complex requirements of the convention.

The CONTINUE convention, however, is different in providing a more
general capability that could be used, for instance, within any other
convention. Deleting a CONTINUE card could in principle change the
meaning of any header in arbitrary ways. I wonder if perhaps we
shouldn't rather be discussing how best to add this capability to the
core FITS standard. Whether convention or new feature of the
standard, however, the loopholes should be closed before adoption.

Rob