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此文曾在本刊1994年第4期发表,由于排版问题,造成一些段落颠倒,现重新刊登.特向作者和读者致歉.
This article has been published in the fourth issue of 1994, due to typesetting problems, resulting in the reversal of some passages, is re-published, especially to the author and readers apologize.