On the street they call it a get-out-of-jail-free card, and drug dealers, con men, and other assorted crooks qualify for it nearly every day.
So what is taking independent counsel Kenneth Starr so long to arrange immunity from prosecution in exchange for the testimony of Monica Lewinsky?
The potential defendant is the president of the United States and Mr. Starr doesn't want to make a mistake, legal experts say.
"When you are investigating a president there is nothing like it, there are no criteria," says Jon Sale, a former Watergate prosecutor who is now a defense attorney and law professor in Miami.
Prosecutors are scrambling to independently verify as much of Ms. Lewinsky's potential testimony as possible. Legal analysts say they are doing this for two reasons. First, to determine the value of her cooperation should they offer her an immunity deal. Second, they want to assess her credibility, whether her testimony will be viewed as voluntary and truthful.
The stakes couldn't be higher.
"She has the potential - depending on what she says - to put the president in a position of having lied," says Stephen Saltzburg, a law professor at George Washington University. "The combination of perjury and lying to the American people is probably going to be enough to end this presidency."
With Clinton supporters, including First Lady Hillary Rodham Clinton, charging that the Starr investigation is part of a right-wing smear campaign, the independent counsel's office is concerned that they not appear to orchestrate the content of Lewinsky's testimony.
At the same time, legal experts say, they must insist that Lewinsky cooperate fully, revealing even those facts that may be the most embarrassing and damaging to the president.
Michael Zeldin, the former independent counsel who investigated the Clinton passport files case, says it is unclear why it is taking so long to reach a cooperation agreement with Lewinsky. "The length of time may reflect some confusion in her story," he says. "I think that her story is short of 'We had an affair and [Vernon] Jordan and the president told me to lie.' "
The key question is are prosecutors pressuring a prospective witness to tell the truth, or are they coaching that witness on what she has to say to qualify for immunity from prosecution. Such coaching is improper and would confirm the fears of some Americans that Starr is conducting a partisan vendetta rather than a search for truth. "They can compel her to tell the truth, they can't compel her to get the president," says Mr. Sale.
Negotiations between Lewinsky's lawyers and the independent counsel's office are shrouded in secrecy. One key issue is whether Lewinsky should receive blanket immunity - in effect a pass for any crime she's committed related to her alleged connection to President Clinton.
Mr. Starr is offering a more limited type of immunity, so-called use immunity. That means that Lewinsky wouldn't be prosecuted for anything she testified about, but could be prosecuted for any crimes later uncovered independent of her testimony.
SUCH 'use immunity' is the norm in the vast majority of federal immunity agreements. Prosecutors prefer use immunity because it creates an incentive for cooperating witnesses to be completely candid to maximize the extent of their protection from prosecution. The same incentive to tell all doesn't exist under the blanket immunity Lewinsky seeks.
Under both types of immunity prosecutors retain the power to charge their immunized witnesses with perjury should they lie while under oath in court.
Lewinsky is seeking immunity out of concern that she could be charged with perjury and subornation of perjury for filing what may have been a false affidavit in the Paula Jones sexual harassment suit and for allegedly urging her friend, Linda Tripp, to file a false affidavit in the same case. If Lewinsky were called to testify without first obtaining an immunity agreement, she would be forced to either admit to a crime or claim her Fifth Amendment privilege against self-incrimination.
"Every person has the right not to give testimony against themselves in a criminal case," says Thomas Sargentich, a law professor at American University's Washington College of Law.
On the other side, prosecutors don't want to lose their leverage over a cooperating witness. While legal analysts say it is extremely rare to prosecute someone for lying in a civil case like the Paula Jones suit, Starr's office must use the threat of prosecution to compel Lewinsky not only to begin cooperating but also to continue to cooperate in the months ahead.