code: def history(self, user=None):
          
          for event in self.changelog:
              when, who, what, old, new, ignore = event
              if (when >= self.options.since.date and
                      when <= self.options.until.date):
                  if user is None or who.startswith(user.login):
                      yield who, what, old, new
docs: Return relevant who-did-what logs from the ticket history