The very incisive editorial "A Mandate for Salinas?," Aug. 26, on the results of the Mexican elections, asks: "Does this mean Mexico's political opening is indeed threatened by renewed PRI preeminence? The forces of economic and political diversity, now set loose, should prevent that."The problem, of course, is that the forces of political diversity have not been set loose. Your own newspaper had, in fact, already given the answer to this problem of political freedom in the article "Will Soviets Learn a Latin Lesson?," Aug. 23, which says: "Populist economic policy has been common enough in Latin America, but analysts say it has never solved the fundamental problems of government that has become too big, too corrupt, and too inefficient." Jorge Prieto, Chicago Letters are welcome. Only a selection can be published, subject to condensation, and none acknowledged. Please address them to "Readers Write," One Norway St., Boston, MA 02115.