The United States Congress came about as the result of a spirited and protracted negotiation; this is an institution that has always had deal-making at its core.
Sometimes that means making arrangements in order to probe the darker recesses of a given political transaction.
Like, say, the relationship between a senior White House official and a foreign power.
In 1857, Congress imported a legal concept from Britain, allowing immunity grants to be conferred upon witnesses to shield them from the legal consequences of testifying.
In those days, Congress could direct a federal judge to order "transactional immunity" – the kind you see in the second half of a Law and Order episode – which had an unintended result: "immunity baths," so dubbed because they washed away one's legal culpability.
As legal expert Hanah Volokh described it in a 2007 Georgetown Law Journal article, "a criminal would convince a friendly member of Congress to call him as a hearing witness and then, with heavy heart, confess all his sins and escape all possible prosecution."
As a result, Congress resorted to conferring a different type of immunity – called "use" immunity – which, in theory, offers more limited protection.
In the intervening decades, use immunity has been offered hundreds of times to entice witnesses to come clean and waive their right against self-incrimination (all that is required is a two-thirds majority vote from a congressional committee).
The power has been invoked during investigations from railway corruption scandals to Watergate – which precipitated another modernization of the immunity statute – to the summary dismissal of federal prosecutors under the George W. Bush administration.
Which brings us to Michael Flynn, the Trump White House's first National Security Advisor. The Wall Street Journal reported this week that a lawyer for the former U.S. Army general has approached various law enforcement agencies and congressional committees seeking an immunity deal to testify about what he knows about the White House and Russia.
"General Flynn certainly has a story to tell, and he very much wants to tell it, should the circumstances permit," his Washington-based attorney Robert Kelner tweeted this week.
There are several good reasons why no one has yet taken him up on the offer.
Many of those are rooted in another pivot point in the history of congressional immunity grants: hearings into a secret program during former president Ronald Reagan's second term to skirt an international arms embargo involving Iran – a bitter geopolitical foe – and divert the proceeds to support anti-communist Contra militias in Nicaragua.
There is a frequent tendency to view the Russia file through the prism of the Watergate break-ins that derailed Richard Nixon's presidency, but the more appropriate analogue may be Iran-Contra – a case that also involved a decorated military official working in the National Security Council, Marine Lt. Col. Oliver North.
"We don't have all the information yet, evidently, but I do see several parallels to the summer of 1987, without question," said Malcolm Byrne, deputy director of the National Security Archive at George Washington University, and author of Iran-Contra: Reagan's Scandal and the Unchecked Abuse of Presidential Power.
Both cases are, at their heart, about the scope of presidential authority.
Mr. Trump may have fired Mr. Flynn last month amid accusations the latter misled the White House about confidential discussions with the Russian ambassador to the United States; this week he defended his former top national security official and even made the eyebrow-raising suggestion on social media he should be granted immunity.
Mr. Reagan and his administration also offered a staunch defence of Mr. North – who, facing jail time, decided to talk – and both presidents have proffered expansive views of executive power.
And in both cases, Congress is being asked to extend immunity to a figure at the epicentre of controversy.
"We do have to be careful though," said Mr. Byrne. "Reagan was still hugely popular, Trump is not. Reagan faced a hostile Congress, Trump does not – although that will be an interesting dynamic to watch given the health-care debacle and the attacks on the House Freedom Caucus."
Then there's the continuing legal fallout from Mr. North's testimony; though use immunity doesn't prevent prosecutors from seeking criminal charges, an appellate panel vacated convictions against Mr. North, saying prosecutors could not plausibly establish that his widely-broadcast congressional testimony (the hearings were a ratings hit) hadn't irreparably tainted the witnesses against him.
Harvard law professor Alex Whiting, in an article for justsecurity.org, argued this week that Mr. Flynn's lawyers could be angling for a similar result.
"The last thing Flynn's lawyer would do if he thought he had the goods would be to go public, because that would potentially compromise the criminal inquiry and would certainly irritate the prosecutors, the very people Flynn's lawyer would be trying to win over," he wrote.
Thus, the public appeal for immunity appears to aimed at Congress, which faces an Iran-Contra-type dilemma; federal law enforcement officials, who are conducting multiple Russia investigations – the latest, according to CBS News, centres on whether Mr. Trump's campaign team may have wittingly or unintentionally assisted Russian hackers in early 2016 – will doubtlessly oppose any proposed immunity grant.
But Mr. Byrne says past precedent suggests an immunity deal will at least be considered.
"Like it or not, [immunity] did get a lot of very good public and secret information into the record," he said. "I'm sure the folks on the Hill are at least thinking about it."
The imponderables in the equation are the partisan dysfunction in Congress and the administrative chaos in the White House.
It's not easy to see how allowing Mr. Flynn to say what he knows under immunity is in the interest of the Republicans who, like during the Iran-Contra period, appear to be in protect-the-party mode.
What isn't clear is whether they're inclined to stake out a similarly robust protect-the-President posture.
The search for truth in Congress is highly politicized, and Mr. Flynn's hopes for an immunity waiver appear dim. That's not the same as saying one or another of the House or Senate committees can't be convinced to use them.
Or that others in the White House orbit won't get one.