The celebrated American lawyer Clarence Darrow once wrote: "History repeats itself and that is one of the things that is wrong with history." He might have added: "Especially in Ireland."
We had as good an example of that as possible when Sir Desmond de Silva published his long-awaited, but already much-abused report on whether security agencies had a hand in the assassination of the Belfast lawyer Patrick Finucane.
We knew that it was gunmen from the Ulster Defence Association who sledgehammered down his door and riddled him with bullets as he prepared to eat Sunday dinner with his family on a February evening in 1989.
But how much did the authorities know beforehand? And did they even connive at the killing of a lawyer regarded by some in government as being "unduly sympathetic to the cause of the IRA"?
The answer from Sir Desmond was a careful "Yes and no". Yes, it was probable that RUC interrogators at Castlereagh Holding Centre had proposed Mr Finucane as a target to UDA detainees.
But no, the allegation that the Army's Force Research Unit (FRU) played a direct role in the murder, knowing from one of its agents in the UDA about the murder plan, but doing nothing to stop it, had no basis in fact. His overall verdict? Not guilty, but not innocent, either.
If the De Silva report was about anything it was less about the RUC's role and more about whether the FRU had a hand in the affair.
And that is because, ever since we came to know of the existence of this supposedly super-secret band of spies and agent-handlers, the FRU has been at the centre of controversy.
The FRU had already been embroiled in allegations which could end political careers and send some of Britain's top spies to jail.
A FRU whistleblower, initially calling himself 'Martin Ingram', but whose real name was Ian Hurst, alleged that a FRU agent at the heart of the IRA's spycatchers unit, codenamed 'Stakeknife' had allowed the IRA to kill people he and the FRU knew were not informers in order to protect real ones.
Then came the charge that a FRU agent at the top of the UDA, its intelligence chief Brian Nelson, had helped set up the murder of Pat Finucane with the FRU's assistance.
The accusation came from Nelson himself and, if Sir Desmond de Silva's report had found the claim to be true, then combined with the 'Stakeknife' allegations, the demand for a thorough public probe of the FRU and its dealings with other secret agencies, like MI5, would have been irresistible.
For some years now, photographs of the FRU have been circulating on the web. One shows nine long-haired young men dressed in civilian clothes and grouped around three cars, each holding a different weapon, which range from pump-action shotguns to heavy machine-guns.
We know these are FRU because one of the nine is Ian Hurst and others in the group have been positively identified as FRU veterans.
There must be something about the lifestyle that tempts such people to pose in such ways, for the FRU pictures are extraordinarily similar to one of the most famous photographs to come out of the Anglo-Irish war of 1919-1921.
That was a photograph purporting to show members of the 'Cairo Gang', an elite group of British Army intelligence officers, recruited - just like the FRU - to achieve the same goal: to defeat the IRA.
And, like the FRU, the Cairo Gang played a central role in the Troubles of that time. The IRA's intelligence chief, Michael Collins, had learned of the group's existence and knew it posed a huge danger so he set out to destroy it.
On a Sunday morning in November 1920, Collins sent out scores of armed IRA men to addresses throughout Dublin with orders to assassinate the Cairo Gang. In the wake of the attacks, the Black and Tans opened fired on a Gaelic football game and, by the end of the day, which became known as Bloody Sunday, more than 30 people lay dead.
The story of one of the dead Cairo Gang members shows how the past can uncannily revisit the future. The dominant figure in the 1920 photograph, standing at the group's centre, is Lieutenant Donald Lewis MacLean, an officer in The Rifle Brigade.
The Rifle Brigade happens to be the regiment where, 25 years later, a young Army officer called Frank Kitson began an illustrious military career.
General Sir Frank Kitson, as he eventually became, was the first Army brigade commander in Belfast of the Troubles and, while in that post, he set up a special military intelligence unit called the Military Reaction Force (MRF) to counter the IRA.
The MRF was also mired in controversy and was wound up, but eventually replaced by a very similar outfit. And guess who its successor was? Yes, the Force Research Unit.
So there it is - a straight line that joins the Cairo Gang of 1920, Michael Collins, Lt Donald Lewis and Bloody Sunday to the Force Research Unit, Frank Kitson, Brian Nelson, 'Stakeknife' and Pat Finucane.
As Clarence Darrow observed, the problem with history is, indeed, that it repeats itself. Except that, in 1920, there was no Rupert Murdoch.
One member of the FRU, Philip Campbell-Smith, has been caught up in the hacking scandal that has plagued Murdoch's News International and has been jailed for stealing information.
His target? None other than fellow FRU veteran Ian Hurst, whose computer was hacked for information about 'Stakeknife' on behalf of the News of the World.
You couldn't make it up. Someone needs to make a movie about the Force Research Unit.