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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 substitute_timestep(self, regex, timestep):
 """ Substitute a new timestep value using regex. """


        # Make one change at a time, each change affects subsequent matches.
        timestep_changed = False
        while True:
            matches = re.finditer(regex, self.str, re.MULTILINE | re.DOTALL)
            none_updated = True
            for m in matches:
                if m.group(1) == timestep:
                    continue
                else:
                    self.str = (self.str[:m.start(1)] + timestep +
                                self.str[m.end(1):])
                    none_updated = False
                    timestep_changed = True
                    break

            if none_updated:
                break

        if not timestep_changed:
            sys.stderr.write('WARNING: no update with {0}.\n'.format(regex))