To persuade thinking persons in Eastern Europe that Central American Marxists - the Sandinistas, the guerillas in El Salvador - are in absurd and tragic error is not difficult. Poles and Czechs and Hungarians can hardly believe, after what they experienced under socialism, that other human beings would fall for the same bundle of lies, half-truths, and distortions. Sadly, however, illusion is often sweeter to human taste than reality. The last marxist in the world will probably be an American nun.