del assumes that A consumers constitute a sufficiently large proportion of the market for B relative to minimum efficient scale to crowd out other producers of B. At the same time, if A consumers represent too large a proportion of the market for B, tying would yield few benefits aside from those that the monopolist could extract simply by charging the monopoly price for A. In addition, the minimum efficient scale must be sufficiently large to facilitate crowding out without being SO large as to render the market for B monopolistic or create efficiency benefits from the elimination of double marginalization. Furthermore, the slope of the average cost must be sufficiently steep to render entry at output levels below minimum efficient scale infeasible. If the slope is relatively shallow, the scale economies will not provide sufficient cost advantage to prevent entry (Yoo 2008). Whinston (1999); see also Carlton and Waldman (2002), similarly concluding that the possibility that exclusion could benefit consumers justifies cautioning against "trying to turn the theoretical possibility for harm shown here into a prescriptive theory of antitrust enforcement." U.S. Steel Corp. V. Fortner Enters., Inc., 429 U.S. 610, 620 (1977); Jefferson Parish Hosp. Dist. No. 2 V. Hyde, 466 U.S. 2, 12-15 (1984); Eastman Kodak Co. V. Image Technical Servs., Inc., 504 U.S. 451, 462, 464, 481 (1992); Ill. Tool Works Inc. V. Indep. Ink, Inc., 547 U.S. 28, 34-5 (2006); accord Nw. Networks in Telecommunications begun to incorporate these structural preconditions into the essential facilities doc- trine. These decisions have always required the practical unavailability of the input from other sources, now formalized into the requirement of proof of a monopoly position in a properly defined market. 30 The need for proof of concentration primary market draws further support from AT&T V. Iowa Utilities Board, in which the Court addressed a statutory provision requiring incumbent local telephone companies to provide competitors with access to any 