No. 86-1448. Argued January 9, 1987. Decided April 15, 1987. Rehearing and Rehearing En Banc Denied May 15, 1987. Marshall Patner, Chicago, Ill., for plaintiffs-appellants. Susan R. Lichtenstein, Schiff, Hardin Waite, Chicago, Ill., for defendants-appellees. Appeal from the United States District Court for the Northern District of Illinois. Before CUMMINGS and FLAUM, Circuit Judges, and ESCHBACH, Senior Circuit Judge. PER CURIAM. Investors in commodities futures contracts must keep "margin" funds