@staticmethod
def _query_const_from_list_subroutine(expr_list):
    """
        A sub method for _query_construction_from_list method in aiding query construction
        :param expr_list: list, relevance query list
        :return: str
        """
    interim_list = []
    interim_str = None
    for index, each_exp_list in enumerate(expr_list):
        for each_exp in each_exp_list:
            interim_list.append(each_exp)
        if index == len(expr_list) - 1:
            interim_str = '({})'.format(' OR '.join(interim_list))
    return interim_str
