AND so, after all the head-scratching mathematics and the slide-rule calculations, it all boiled down to something fierce and elemental: Leinster against Munster. Irish province against Irish province.
The reigning champions fulfilled their part of the bargain in the biting cold of the south of England yesterday, putting a gallant Exeter to the sword and racking up four tries in the process. They would have liked more, but it will have to do.
Nobody expected Exeter to roll over and die, of course, but such was the spikiness of their resistance that Leinster should feel happy to have escaped with their tournament lives still in reasonable enough shape.
There were times when it looked as if their fitness and their pedigree might help them gallop clear, but Exeter never took long in disabusing us of such notions. In the end, Joe Schmidt's team fell over the line in a relieved heap.
They left the ball in Munster's court now, as they wanted to do. The scrap for the eighth quarter-final berth may yet take another twist depending on how other games go today but, fundamentally, a bonus edging their fierce rivals for the last coveted spot. The pool stages, surely, have never delivered a more thrilling climax.
It went without saying yesterday that Leinster needed a quick start and, to their relief, it came. When they had worked a couple of early casual errors out of their system, they came charging at Exeter with ruinous intent. Only three minutes had elapsed when Gordon D'Arcy cut inside to score when it looked just as easily as if their forwards might have rampaged over a phase or two later.
It was a deceptively easy score for Leinster. Deceptive because the Chiefs quickly reminded us they had no intention of folding tamely on their own ground, as they had against Clermont last October.
Luke Arscott might have caused some damage had he managed to hold onto a Kevin Barrett pass when arriving at speed and then Leinster, through rousing carries from Richardt Strauss and Cian Healy, threatened at the other end. Exeter held firm.
It was a game played at a frantic pace and it was never clear whether that was in Leinster's favour or not. For a team needing tries — and lots of them — the open nature of the exchanges should have been ideal, but that was to factor Exeter out of that equation.
For the sheer abandon with which they played, the desire to end their maiden Heineken Cup campaign on a high and, if possible, take a major scalp, Exeter were a joy to watch.
And so they weren't at all flattered when they squared the match after 16 minutes, as ugly as the concession was from Leinster's perspective, Gareth Steenson kicking them into the 22 after Rob Kearney was caught dithering in midfield and then, conveniently, finding a huge hole in Leinster's defence to push Neil Clark through for a try and Steenson coolly added the two points.
For Leinster the realisation that they couldn't beat Exeter for power came early. However they would achieve their target, it was clear that Exeter simply wouldn't be bullied into submission. As the game approached the half-hour mark Leinster again found themselves camped on the fivemetre line but two massive scrums failed to go their way.
The Exeter front-eight simply refused to buckle.
So Leinster fell back on the more traditional route instead.
Jonathan Sexton picked up from the bottom of a ruck and swept a pass out to Brian O'Driscoll, far from perfect, but the recently deposed Irish captain showed beautiful hands to clasp it and toss to Kearney in one fluid movement and the fullback atoned for his earlier lapse by skipping inside Matt Jess's despairing lunge to touch down. Sexton missed the conversion, but the day was still about tries rather than points.
While they kept producing such moments of beauty, Leinster always had hope. The problem was, though, that Exeter were playing to a more destructive script. They were on top in the scrum, comfortable in the line-out and caused enough destruction that Leinster, humiliatingly, had conceded a penalty try and a Steenson penalty by the interval. Exeter, incredibly, led by five.
Leinster had lost Strauss to injury by then and things started to look hairy. It was the type of game, though, in which a twist never seemed far away.
Four minutes after the restart, the game turned again, Cullen producing a neat offload to put O'Driscoll in the clear. A few minutes later their forwards looked like they might power Exeter back over their own line but, again, the Chiefs held firm.
You wondered how long they could keep the growing Leinster tide at bay, though.
Not long as it happened.
Leinster's fourth try was a cruel insult to all that had gone before. Exeter had a put-in inside their own '22' but, for once, they were shunted backwards and coughed up possession.
From the next set-piece Jamie Heaslip did nothing more than pick up from the base of the scrum and charge past a line of Exeter defenders unable to intercede. As brutally simple as that.
Yet there were still 28 minutes to go and Leinster could neither add to their try tally or achieve any sense of comfort.
Steenson kicked a penalty with 10 minutes remaining, Isaac Boss shipped a yellow card — joining Exeter's Ben White in the bin — and the home side still had their dander up. But Sexton restored the balance to nine points and Leinster played the game out, settling for what they had. So they left for home having done enough to maintain a pulse, but not much more than that, their destiny frustratingly in Munster hands.