Identifier     add_history_string

 Purpose        Add a given string to the history string of a given structure
                in the format of ISAP history handling

 Synopsis       add_history_string, struct, newstring, xx, stat

 Arguments      Name      I/O  Type      Description
                -----------------------------------------------------
                struct    I/O  struct    The structure containing the history
                                         tag to be modified
                newstring  I   string    The string to be written 
                xx         I   string    The two character code describing the 
                                         nature of the entry (see description
                                         of update_history)
                stat       O   integer   error status, 0 if ok, 1 if fatal

 Returns        ---

 Description    If history is empty or does not contain an END line fill in 
                the END line.
                Check if history containes only 80 character lines. If not, 
                fill up with zeros.
                See if history string will get longer than 32767 characters.
                If so expand the dimension of the history tag.
                See if new string is longer than 64 characters and has to be
                broken.
                Add new string to history.

 Comment        This routine will usually be called from update_history. To
                add a string interactively to the history use either
                update_history or write2hist.

 Example        ---

 Dependencies   CALLS: sap_error
                CALLED FROM: update_history.pro, write2hist.pro

 Category       ISAP

 Filename       add_history_string.pro

 Author         E. Sturm (--> ES)

 Version        1.1

 History        0.1   26-10-95  ES    
                1.0   30-11-95  ES  V1 delivery
                1.1   13-05-96  ES  error status added
                      11-07-96  ES  slight modification of stat
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Copyright (C) 1995, Max-Planck-Institut fuer extraterrestrische Physik (MPE);
                    Garching, Germany                                       ;
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