prevent a judgment, unreviewable because of mootness, from spawning any legal consequences." U.S. v. Munsingwear, 340 U.S. 36, 41 (1950), cited in O'Connor v. Donaldson, 422 U.S. 563, 577 n. I2 (1975) (vacatur "deprives [the lower] court's opinion of precedential effect"); see also Associated Gen. Contractors of Con