code: def get_session_identifiers(cls, folder=None, inputfile=None):
          
          sessions = []
          if inputfile and folder:
              raise MQ2Exception(
                  'You should specify either a folder or a file')
          if folder:
              if not os.path.isdir(folder):
                  return sessions
              for root, dirs, files in os.walk(folder):
                  for filename in files:
                      filename = os.path.join(root, filename)
                      for ext in SUPPORTED_FILES:
                          if filename.endswith(ext):
                              wbook = xlrd.open_workbook(filename)
                              for sheet in wbook.sheets():
                                  if sheet.name not in sessions:
                                      sessions.append(sheet.name)
          elif inputfile:
              if os.path.isdir(inputfile):
                  return sessions
              for ext in SUPPORTED_FILES:
                  if inputfile.endswith(ext):
                      wbook = xlrd.open_workbook(inputfile)
                      for sheet in wbook.sheets():
                          if sheet.name not in sessions:
                              sessions.append(sheet.name)
          return sessions
docs: Retrieve the list of session identifiers contained in the
          data on the folder or the inputfile.
          For this plugin, it returns the list of excel sheet available.
  
          :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