This episode's headline: Farm Depression. The guests include Rep. THOMAS DASCHLE, Democrat, South Dakota; JOHN BLOCK, Secretary of Agriculture; In Lincoln, Nebraska (Facilities: Nebraska ETV Network): TOM NICHOLS, Farmer. Byline: In New York: ROBERT MacNEIL, Executive Editor; In Washington: JIM LEHRER, Associate Editor; LEWIS SILVERMAN, Producer; MARIE MacLEAN, Rporter