[fitsbits] Clarification of EXTEND, please

Stephen Walton writes:
Without naming names, I just today hit a conflict between two pieces of
software. One created a FITS file with only a single HDU whose header
contained the line 'EXTEND = F'. A second piece of software interpreted
the presence of the EXTEND keyword in the primary HDU as meaning it
should check for extensions, tried to fseek() beyond the end of the
primary HDU and data array, and died.

Who did what right and wrong in this scenario? I'll send bug reports
with the correct answers to the software authors.

The second piece did wrong, twice :-) EXTEND indicates only that the file
may contain an extension, not that it does, and then only when set to T.
It looks like they took two unwarranted shortcuts...