Identifier   write_hdrkey

 Purpose      Write a keyword value and optionally the corresponding comment
              into the header of an ISAP structure

 Synopsis     write_hdrkey, header, keyword, value, status [,comment=comment] 
                          [,pos=pos] [,second=second] [,errstat=errstat]

 Arguments    Name      I/O Type    Description
              -----------------------------------------------------
              header     I  string  The input header 
              keyword    I  string  Keyword to write into header
              value      I  varies  The value of the keyword. Allowed values:
                                    integer or long, float or double, string,
                                    and 0B for logical false or 1B for 
                                    logical true.
              status     O  integer status code
                                    0 keyword inserted as a new header line
                                    1 keyword existed, overwritten
                                    2 like 1, but old and new type differ
              comment    I  string  The comment for the keyword
              pos                   The position at which the new keyword
                                    shall be inserted. Ignored if keyword 
                                    already exists
              second                If set the new keyword is written 
                                    directly before the END line
                                    of the secondary header, else directly
                                    before the END line of the primary header
                                    (default). Ignored if pos is set or if
                                    keyword is found in primary header
              errstat   I/O integer error status, 0 if ok, 1 if fatal

 Returns      

 Description  Writes a keyword into a header either before the end of the
              primary header (default) or before the end of the secondary
              header (if /second is set) or at the given position (if pos
              is given). If the keyword is already present, it is overwritten
              At the moment the first occurence of the keyword is taken.
              So the primary and secondary MUST NOT HAVE IDENTICAL KEYWORDS! 
              Ensures that the product has correct length (blocks
              of 2880 characters) according to the fits standard.
              In case of a failure the program returns to the calling level
              and the header is not overwritten

 Comment      

 Example        1) header = aar.header
                   write_hdrkey, header, 'TUNIT1', 'cm', stat
                   if stat eq 1 then aar.header = header
                This will overwrite the TUNIT1 value in the secondary header

 Dependencies   CALLS: get_hdrkey, sap_error, sap_keyword_set
                CALLED FROM: ascii2yaaar, spect_form, wave_form, wav_dop

 Category     ISAP

 Filename     write_hdrkey.pro 

 Author       E. Sturm  (--> ES)

 Version      1.1

 History      1.0   25-01-96  ES  First version
              1.1   10-07-96  ES  errstat added

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Copyright (C) 1995, Max-Planck-Institut fuer extraterrestrische Physik (MPE);
                    Garching, Germany                                       ;
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