"The rule is the lock-up option precludes pooling that would trigger the exercisability of the option," Mr. Willens of Lehman Brothers said.

"Apparently, the only way the lock-up option can be rescinded is if the court decides the lock-up option was illegal from the outset and strikes it down."

In a letter to Holly's board, Tyson's chairman and chief executive, Don Tyson, raised the company's latest offer from $60 a share and conditioned it upon Holly ending a lock-up option with Conagra Inc., the Omaha-based food products giant.

This case involved a proposed merger between Viacom and Paramount Pictures; as part of the merger agreement, Paramount agreed to an array of defensive measures, including a no-shop provision, $100 million termination fee and a lock-up option on approximately 20% of Paramount's common stock.