def get_session_identifiers(cls, folder=None, inputfile=None):
        """ 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

        """
        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
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