docs: def get_session_identifiers(cls, folder=None, inputfile=None):
          """ Retrieve the list of session identifiers contained in the
          data on the folder.
  
          :kwarg folder: the path to the folder containing the files to
              check. This folder may contain sub-folders.
          :kwarg inputfile: the path to the input file to use
  
          """
          sessions = []
          if folder is None or not os.path.isdir(folder):
              return sessions
          for root, dirs, files in os.walk(folder):
              for filename in files:
                  if filename.startswith('Session ') \
                          and filename.endswith('.mqo'):
                      session = filename.split()[1]
                      if session not in sessions:
                          sessions.append(session)
          return sessions