NAME
     replacekey - replace placeholder in a FITS header

SYNOPSIS
     replacekey <in> <card> [placeholder]

DESCRIPTION
     replacekey is aimed at replacing placeholder cards in a FITS file's
     header by cards provided by the user on the command-line. The idea is
     to decouple the process doing the actual image processing on the data
     from the process that needs to update the FITS header. To do this, it
     is enough to place "blank" cards (or placeholders) in the files
     produced by the image processing routine, and update the output header
     with this utility. The default placeholder is a comment card as:
     COMMENT PLACEHOLDER

     You can change the name of the placeholder by providing it (in quotes)
     on the command-line as third argument. You need to provide the name of
     the input FITS file you want to modify, the value to place in the
     header, and optionally this placeholder name. You can actually use
     replacekey to do plain search and replace in a FITS header, by
     providing any character string as placeholder. The first card matching
     the string you provide will be replaced.

     Examples:

     You want to place a card containing VALUE=32 in the header, in place
     of the default placeholder (COMMENT PLACEHOLDER), in the file named
     myfile.fits:
     % replacekey myfile.fits "VALUE  = 32"

     If your do not care about the HISTORY cards you have in the header,
     and want to replace the first one by VALUE=32, you would do:
     % replacekey myfile.fits "VALUE  = 32" "HISTORY"

     If you want to replace the previous VALUE=32 by VALUE=64, you would
     simply do:
     % replacekey myfile.fits "VALUE  = 64" "VALUE"

     Notice that the character string you give as second argument will be
     placed verbatim in the header. If you provide more than 80 characters,
     they will be truncated to the first 80 characters. If you provide less
     characters, blanks will be added up to character 80. You should be
     careful when adding cards, that they respect the FITS format. Nothing
     prevents you from doing:
     % replacekey myfile.fits "VALUE= 1 2 3 4 /" "HISTORY"

     which will replace the first occurence of a HISTORY card by the line
     you provided, but the replacing line is not FITS-compliant. No tests
     are performed internally to check the compliance of the FITS line you
     are adding.

OPTIONS
     None

SEE ALSO
     hierarch28 to convert HIERARCH ESO keywords to regular 8-char
     keywords.

FILES
     Files shall all comply with FITS format

BUGS
     Modifications are so far only possible in the main header, not in the
     extensions. That will be added later.

AUTHOR
     N. Devillard - ESO