[fitsbits] once FITS always FITS

[Steve Allen]

On Tue 2007-08-21T00:07:29 -1000, Maren Purves hath writ:
> It being "once FITS always FITS" I object to making the
> EXTEND keyword optional.

A question for those who have been around longer than I:

Does "once FITS always FITS" mean

  Anything ever explicitly allowed can never be disallowed
or
  Anything ever explicitly allowed must always remain required
or
  Anything not explicitly disallowed must always remain allowed
or
  Anything ever explicitly disallowed can never be allowed
or some other rearrangment of those notions, or what?

In RFC 4047 we have
  no change may be made to FITS that invalidates existing files
but I'm not sure that's what everyone thinks.

--
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