SXADDPAR

       Add or modify a parameter in a FITS header array.

       SXADDPAR, Header, Name, Value, [ Comment,  Location, /SaveComment,
                               BEFORE =, AFTER = , FORMAT= , /PDU]

       Header = String array containing FITS or STSDAS header.    The
               length of each element must be 80 characters.    If not
               defined, then SXADDPAR will create an empty FITS header array.
       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 'COMMENT'
               or 'HISTORY' or a blank string then the value will be added to
               the record without replacement.  For these cases, 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
               '', or /SAVECOMMENT is set, then the previous comment field is
               retained (when found)
       Location = Keyword string name.  The parameter will be placed before the
               location of this keyword.    This parameter is identical to
               the BEFORE keyword and is kept only for consistency with
               earlier versions of SXADDPAR.

       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.  If not supplied then the default is
               'G19.12' for double precision, and 'G14.7' for floating point.
       /PDU    = specifies keyword is to be added to the primary data unit
               header. If it already exists, it's current value is updated in
               the current position and it is not moved.
       /SAVECOMMENT = if set, then any existing comment is retained, i.e. the
               COMMENT parameter only has effect if the keyword did not
               previously exist in the header.

       Header = updated FITS header array.

       Add a keyword 'TELESCOP' with the value 'KPNO-4m' and comment 'Name
       of Telescope' to an existing FITS header h.
       IDL> sxaddpar, h, 'TELESCOPE','KPNO-4m','Name of Telescope'

       The functions SXADDPAR() and FXADDPAR() are nearly identical, with the
       major difference being that FXADDPAR forces required FITS keywords
       BITPIX, NAXISi, EXTEND, PCOUNT, GCOUNT to appear in the required order
       in the header, and FXADDPAR supports the OGIP LongString convention.
       There is no particular reason for having two nearly identical
       procedures, but both are too widely used to drop either one.
       All HISTORY records are inserted in order at the end of the header.
       All COMMENT records are also inserted in order at the end of the header
       header, but before the HISTORY records.  The BEFORE and AFTER keywords
       can override this.
       All records with no keyword (blank) are inserted in order at the end of
       the header, but before the COMMENT and HISTORY records.  The BEFORE and
       AFTER keywords can override this.

       Warning -- Parameters and names are not checked
               against valid FITS parameter names, values and types.

       DMS, RSI, July, 1983.
       D. Lindler Oct. 86  Added longer string value capability
       Converted to NEWIDL  D. Lindler April 90
       Added Format keyword, J. Isensee, July, 1990
       Added keywords BEFORE and AFTER. K. Venkatakrishna, May '92
       Pad string values to at least 8 characters   W. Landsman  April 94
       Aug 95: added /PDU option and changed routine to update last occurence
               of an existing keyword (the one SXPAR reads) instead of the
               first occurence.
       Comment for string data can start after column 32 W. Landsman June 97
       Make sure closing quote supplied with string value  W. Landsman  June 98
       Converted to IDL V5.0    W. Landsman   June 98
       Increase precision of default formatting of double precision floating
               point values.   C. Gehman, JPL  September 1998
       Mar 2000, D. Lindler, Modified to use capital E instead of lower case
               e for exponential formats.
       Apr 2000, Make user-supplied format upper-case  W. Landsman
       Oct 2001, Treat COMMENT or blank string like HISTORY keyword W. Landsman
       Jan 2002, Allow BEFORE, AFTER to apply to COMMENT keywords W. Landsman
       June 2003, Added SAVECOMMENT keyword    W. Landsman
       Jan 2004, If END is missing, then add it at the end W. Landsman
       May 2005 Fix SAVECOMMENT error with non-string values W. Landsman

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