In Blonder-Tongue Laboratories, Inc. v. University of Illinois Foundation, 402 U.S. 313, 91 S.Ct. 1434, 28 L.Ed.2d 788, 169 USPQ 513 (1971), the Supreme Court ruled that once the claims of a patent are held invalid in a suit involving one alleged infringer, an unrelated party who is sued for infringement of those claims may reap the benefit of the invalidity decision under principles of collateral estoppel. Mutuality of estoppel is no longer required. Thus, the benefits of collateral estoppel (now generally termed issue preclusion) arising from a final judgment of patent invalidity were extended to an alleged infringer other than the defendant who earlier successfully litigated the matter and those in privity therewith. Under Blonder-Tongue, 402 U.S. at 333, 91 S.Ct. at 1445, the Court recognized this countervailing argument to acceptance of collateral estoppel in a particular case. Whether a patentee has had a full and fair opportunity to litigate may be a difficult decision in some instances, and "no one set of facts, no one collection of words or phrases will provide an automatic formula for proper rulings on estoppel pleas." 402 U.S. at 334, 91 S.Ct. at 1445.