This episode's headline: Nicaragua: Contra Aid; Olympian Security; Tax Shelters; Campaigning on Public Funds; Eudora Welty at 75. The guests include In Washington: Lt. Col. JOHN BUCHANAN, U.S. Marine Corps, Ret.; In New York: JEROME KURTZ, Former IRS Commissioner. Byline: In New York: ROBERT MacNEIL, Executive Editor; CHARLAYNE HUNTER-GAULT, Correspondent; In Washington: JUDY WOODRUFF, Correspondent; Reports from NewsHour Correspondents: KEITH GRAVES, in Israel; JUNE MASSELL, in Los Angeles; NANCY NICHOLS, in Jackson, Mississippi