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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _write_history(self, pfx, hdu, linechar=60, indentchar=2):
 """Write change history to given HDU header. Limit each HISTORY line to given number of characters. Subsequent lines of the same history will be indented. """

        channel = self.fv.get_channel(self.chname)
        if channel is None:
            return

        history_plgname = 'ChangeHistory'
        try:
            history_obj = self.fv.gpmon.getPlugin(history_plgname)
        except Exception:
            self.logger.error(
                '{0} plugin is not loaded. No HISTORY will be written to '
                '{1}.'.format(history_plgname, pfx))
            return

        if channel.name not in history_obj.name_dict:
            self.logger.error(
                '{0} channel not found in {1}. No HISTORY will be written to '
                '{2}.'.format(channel.name, history_plgname, pfx))
            return

        file_dict = history_obj.name_dict[channel.name]
        chistory = []
        ind = ' ' * indentchar

        # NOTE: List comprehension too slow!
        for key in file_dict:
            if not key.startswith(pfx):
                continue
            for bnch in file_dict[key].values():
                chistory.append('{0} {1}'.format(bnch.MODIFIED, bnch.DESCRIP))

        # Add each HISTORY prettily into header, sorted by timestamp
        for s in sorted(chistory):
            for i in range(0, len(s), linechar):
                subs = s[i:i + linechar]
                if i > 0:
                    subs = ind + subs.lstrip()
                hdu.header.add_history(subs)