1 be substantially to lessen competition, or to tend to create a monopoly." 15 U.S.C.A. § 1 (1981). Mergers subject to section 1 are prohibited if they constitute a 2 “contract, combination... or conspiracy in restraint of trade." They replace a set of Guidelines issued by the Department in 1968, and are subject to 3 further revision in light of subsequent judicial decisions or economic studies...