ed above, but also require that certain assumptions within a very narrow range. 26 Furthermore, Whinston found that even when vertical exclusion was possible, the welfare implications were ambiguous. Whinston accordingly concluded that the normative implications of his model were unclear and that even when tying led to vertical exclusion, permitting it might actually benefit consumers. 27 Consequently, any court considering an antitrust claim based on the exception identified by Whinston must ensure that its restrictive assumptions are met and should undertake a complete analysis of the welfare implications before imposing liability. Over time, antitrust law began to reflect the characterization of vertical exclu- sion found in the scholarly commentary. Of particular relevance for our discus- sion are the Court's most recent tying decisions, which like the essential facilities decisions are based on unilateral conduct and thus fall within Section 2. In those decisions, the Supreme Court has begun to require proof that the structural precon- ditions discussed above are met before finding liability. For example, the Supreme Court has required that a tying defendant possess monopoly power in a properly defined market. 28 Lower courts have followed the Supreme Court's lead and have The primary scenario Whinston analyzes assumes that the market for good A is a monopoly, the market for good B is characterized by economies of scale, and consumers of good A represent significant percentage of consumers of good B. For example, assume that 75% of the consumers of good B also purchase good A and that the minimum efficient scale for the market for B is 25%. Tying B to A forces all other producers of good B below minimum efficient scale and can therefore give the firm producing A a monopoly over B as well. In the process, the A monopolist is able to exert monopoly power over the 25% of the consumers of B who do not purchase A. Whinston (1990); see also Carlton and Waldman (2002), extending Whinston's work. For example, the mo