[fitsbits] Definition of "deprecated"

On Mon 2007/08/27 10:19:32 MST, "Dick Shaw" wrote
in a message to:

"This term is used to refer to obsolete structures that _should not_ be used
for new applications but _shall_ remain valid for use by applications that
already use the deprecated structure."
:
"This term refers to obsolete features or structures in FITS that _should
not_
be used in new FITS files, but _shall_ remain valid indefinitely. FITS users
and data engineers should be aware that the use of deprecated structures is
considered bad form and should be avoided."

This wording is a great improvement but still it doesn't quite capture
the dichotomy between FITS writers and FITS readers, new and old. There
are two types of deprecation:

1) Constructs that ought never be used by writers (but must still be
understood by readers) e.g. omitting the "/" comment delimiter,
or implicit decimal points in ASCII table fields. It is safe to
outlaw such usage because doing so has no effect on old readers.

2) Constructs that shouldn't be used by writers but might still be in
order to help old readers (and must still be understood by new
readers). EXTEND, EPOCH and CROTAn are examples.

Moreover, where any usage is deprecated it must still be fully described
in the spec in order for FITS readers to be able to handle the existing
body of old FITS files (and of course, old FITS writers will carry on
doing what they've always done).

Mark Calabretta
ATNF