FXHMODIFY

       Modify a FITS header in a file on disk.

       Opens a FITS file, and adds or modifies a parameter in the FITS header.
       Can be used for either the main header, or for an extension header.
       The modification is performed directly on the disk file.

       FXHMODIFY, FILENAME, NAME, VALUE, COMMENT

       FILENAME = String containing the name of the file to be read.
       NAME    = Name of parameter.  If NAME is already in the header the
                 value and possibly comment fields are modified. Otherwise a
                 new record is added to the header.  If NAME is equal to
                 either "COMMENT" or "HISTORY" then the value will be added to
                 the record without replacement.  In this case the comment
                 parameter is ignored.
       VALUE   = Value for parameter.  The value expression must be of the
                 correct type, e.g. integer, floating or string.  String
                 values of 'T' or 'F' are considered logical values.

       COMMENT = String field.  The '/' is added by this routine.  Added
                 starting in position 31.  If not supplied, or set equal to ''
                 (the null string), then any previous comment field in the
                 header for that keyword is retained (when found).

       None.

       None.

       EXTENSION = Either the number of the FITS extension, starting with the
                   first extension after the primary data unit being one; or a
                   character string containing the value of EXTNAME to search
                   for.  If not passed, then the primary FITS header is
                   modified.
       BEFORE  = Keyword string name.  The parameter will be placed before the
                 location of this keyword.  For example, if BEFORE='HISTORY'
                 then the parameter will be placed before the first history
                 location.  This applies only when adding a new keyword;
                 keywords already in the header are kept in the same position.
       AFTER   = Same as BEFORE, but the parameter will be placed after the
                 location of this keyword.  This keyword takes precedence over
                 BEFORE.
       FORMAT  = Specifies FORTRAN-like format for parameter, e.g. "F7.3".  A
                 scalar string should be used.  For complex numbers the format
                 should be defined so that it can be applied separately to the
                 real and imaginary parts.
       ERRMSG  = If defined and passed, then any error messages will be
                 returned to the user in this parameter rather than
                 depending on the MESSAGE routine in IDL.  If no errors are
                 encountered, then a null string is returned.  In order to
                 use this feature, ERRMSG must be defined first, e.g.
                       ERRMSG = ''
                       FXHMODIFY, ERRMSG=ERRMSG, ...
                       IF ERRMSG NE '' THEN ...

       FXHREAD, FXPAR, FXADDPAR, BLKSHIFT

       None.

       This routine can not be used to modify any of the keywords that control
       the structure of the FITS file, e.g. BITPIX, NAXIS, PCOUNT, etc.  Doing
       so could corrupt the readability of the FITS file.

       If adding a record to the FITS header would increase the
       number of 2880 byte records stored on disk, then the file is
       enlarged before modification, unless the NOGROW keyword is passed.

       Data Handling, I/O, FITS, Generic.

       None.

       William Thompson, GSFC, 3 March 1994.

       Version 1, William Thompson, GSFC, 3 March 1994.
       Version 2, William Thompson, GSFC, 31 May 1994
               Added ERRMSG keyword.
       Version 3, William Thompson, GSFC, 23 June 1994
               Modified so that ERRMSG is not touched if not defined.

       Version 3, 23 June 1994
       Converted to IDL V5.0   W. Landsman   September 1997

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