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The following strong recommendations are made regarding keyword usage:
- FITS data files should include the checksum keywords in every HDU.
- FITS software should update the checksum keywords if present,
or add the keywords if not present.
- Software that modifies FITS files should at the very least
delete the keywords if no other checksum handling is implemented.
- All HDUs of a single FITS file should use the same
The intent of these recommendations is to avoid a situation in which a
FITS file has been modified, and the checksum allowed to become
out of date.