??? ????? ??????? ??????? ???? ?? ???????? ???????? ?????????? ????????? ???????ɡ ???????? ????????? ????ɡ ???? ????? ???? ?? ???????? ????????? ??? ?????? ????? ?????? ???????? ??? ?????? ?????. ??? ??????? ?????? ??? ??????? ??????? (1982)? ??????? ??? ????? ????? ?????? (1980) ??????? ??? ??????? ??????? (1992).