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[5]This restriction is currently
necessary since the OGIP format for PHA files (as described in Arnaud et al)
allows columns which have the same value in all rows to he replaced by a
keyword (of the same name) in the extension header. Such a situation often
arises for PHA datasets for which the grouping, systematic error or quality
flags have not yet been set. Thus in the general case in which there are
additional extensions within the FITS file following that containing the PHA
dataset, it cannot be guaranteed that there will he enough room within the
existing file to add all the extra information (grouping and quality flags,
systematic errors) which can he set within GRPPHA. Furthermore, since GRPPHA
does not read and store all data from any other extensions within the input
file (but simply copies them when a new file is to be written), the input file
cannot be removed upon input. However, due to user pressure, methods by which
the input file can he overwritten are under investigation and this restriction
may he lifted in the near future.
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