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      Add or modify a parameter in a FITS header array.


      This version of FXADDPAR will write string values longer than 68
      characters using the FITS continuation convention described at




      HEADER = String array containing FITS header. The maximum string
                length must be equal to 80. If not defined, then FXADDPAR
                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
                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 unless the /NOLOGICAL keyword is set. If the value is
                a string and is "long" (more than 69 characters), then it
                may be continued over more than one line using the OGIP
                CONTINUE standard.
  Opt. Inputs :
      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).


      HEADER = Updated header array.
  Opt. Outputs:


      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
      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 IDL
                default formatting is used, except that double precision is
                given a format of G19.12.
      /NOCONTINUE = By default, FXADDPAR will break strings longer than 68
                characters into multiple lines using the continuation
                convention. If this keyword is set, then the line will
                instead be truncated to 68 characters. This was the default
                behaviour of FXADDPAR prior to December 1999.
      /NOLOGICAL = If set, then the values 'T' and 'F' are not interpreted as
                logical values, and are simply added without interpretation.
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, e.g.


  Common :


      Warning -- Parameters and names are not checked against valid FITS
      parameter names, values and types.
      The required FITS keywords SIMPLE (or XTENSION), BITPIX, NAXIS, NAXIS1,
      NAXIS2, etc., must be entered in order. The actual values of these
      keywords are not checked for legality and consistency, however.

Side Effects

      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, 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.
      All other records are inserted before any of the HISTORY, COMMENT, or
      "blank" records. The BEFORE and AFTER keywords can override this.
      String values longer than 68 characters will be split into multiple
      lines using the OGIP CONTINUE convention, unless the /NOCONTINUE keyword
      is set. For a description of the CONTINUE convention see


      Data Handling, I/O, FITS, Generic.
  Prev. Hist. :
      William Thompson, Jan 1992, from SXADDPAR by D. Lindler and J. Isensee.
      Differences include:
              * LOCATION parameter replaced with keywords BEFORE and AFTER.
              * Support for COMMENT and "blank" FITS keywords.
              * Better support for standard FITS formatting of string and
                complex values.
              * Built-in knowledge of the proper position of required
                keywords in FITS (although not necessarily SDAS/Geis) primary
                headers, and in TABLE and BINTABLE extension headers.
      William Thompson, May 1992, fixed bug when extending length of header,
      and new record is COMMENT, HISTORY, or blank.
  Written :
      William Thompson, GSFC, January 1992.
  Modified :
      Version 1, William Thompson, GSFC, 12 April 1993.
              Incorporated into CDS library.
      Version 2, William Thompson, GSFC, 5 September 1997
              Fixed bug replacing strings that contain "/" character--it
              interpreted the following characters as a comment.
      Version 3, Craig Markwardt, GSFC, December 1997
              Allow long values to extend over multiple lines
Version 4, D. Lindler, March 2000, modified to use capital E instead
of a lower case e for exponential format.
      Version 4.1 W. Landsman April 2000, make user-supplied format uppercase
      Version 4.2 W. Landsman July 2002, positioning of EXTEND keyword
      Version 5, 23-April-2007, William Thompson, GSFC
      Version 6, 02-Aug-2007, WTT, bug fix for OGIP long lines
      Version 6.1, 10-Feb-2009, W. Landsman, increase default format precision
      Version 6.2 30-Sep-2009, W. Landsman, added /NOLOGICAL keyword


      Version 6.2, 30-Sep-2009

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