docs: def checkMaxSessions(self, nMax=None):
          """
          check whether max. number of saved sessions is reached
          if: remove the oldest session
          """
          if nMax is None:
              nMax = self.opts['maxSessions']
          l = self.stateNames()
          if len(l) > nMax:
              for f in l[:len(l) - nMax]:
                  self.tmp_dir_session.remove(str(f))