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SECTION 4. HEADERS

for the highest level; levels with a higher value of this keyword shall be subordinate to

levels with a lower value. If the EXTLEVEL keyword is absent, the file should be treated

as if the value were 1.

4.4.3

Additional Keywords

New keywords may be devised in addition to those described in this standard, so long

as they are consistent with the generalized rules for keywords and do not conflict with

mandatory or reserved keywords. Any keyword that refers to or depends upon the

existence of other specific HDUs in the same or other files should be used with caution

because the persistence of those HDUs cannot always be guaranteed.

FITS Standard