How do you solve a problem like the Scarlets?
That was a conundrum that neither Leinster nor Munster could crack last season in the matches that mattered.
Twice in the space of eight days last May the Welsh side came to Dublin and comprehensively beat the top two Irish provinces.
They played most of the semi-final against Leinster at the RDS with 14 men after Steff Evans was red-carded in the 37th minute.
They won 24 turnovers during the game and had three tries on board after half an hour.
It was free-flowing, quick-hands rugby and Leinster were stunned, becoming the first home side to lose a semi-final in the competition.
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"We came well and truly second place against Scarlets last year," Leo Cullen told RTÉ Sport after yesterday’s quarter-final win over back-to-back Champions Cup winners Saracens that set up a semi-final against the Welsh team.
"That was with Scarlets having 14 men for a chunk of the game so we know it's going to be a tough challenge."
That’s putting it mildly.
Munster had been warned but Munster could do nothing about it when Scarlets reproduced a similar effort in the Aviva winning the final by a record margin, 46-22.
This time they had four tries by the 30-minute mark and the game was over as a contest at that stage.
To their number they’ve added ‘Laser’ Leigh Halfpenny, while Munster-bound Tadhg Beirne is playing exceptional rugby and has earned a European player of the year nominee, he gets better every game.
They topped Pool 5 of the Champions Cup, beating Toulon along the way. They were excellent value for their 29-17 quarter-final win over La Rochelle, the standout team of the early pool stages.
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As ably demonstrated last year, travelling over to Dublin and facing the locals bothers them not a jot.
"Scarlets will have no fear," recognised Johnny Sexton. They should sing that for the next 20 days.
The absence of doubt is a powerful weapon in any arsenal.
The teams met twice over the course of the Six Nations, those games producing a 10-10 draw in Llanelli and a seven-point win for the Blues in Dublin.
"We played Scarlets twice during the Six nations, two very close games. I know that both teams were missing a lot of players but Scarlets are a team that caused a hell of a lot of trouble so we've full respect for them.
"The players will be well and truly focused when that game comes around."
There’ll be nobody to claim that they weren’t warned should Leinster fail to bring anything less than their ‘A’ game to the Aviva.
"They definitely got one over us in the RDS last year," said captain Isa Nacewa, who still feels the hurt of their other semi-final defeat last year, the Champions Cup loss away to Clermont.
"They’re such a good attacking side these days and they showed that the other evening.
"They’re a really tough challenge and one we’ll have to learn from today and really build for.
"They are a huge challenge, one of the best attacking teams out there.
"They thrive off turnover ball, [and have] one of the highest rates of turning over ball and converting those into points.
"All the guys they have, they can play to both game plans and they are a very expansive team as we know.
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"It was a tough pill to swallow, losing a semi-final as we did away last year to Clermont.
"That has been etched in the back of our head and it is something we don’t like feeling. We have talked about it a lot, and off-season we have learned from it.
"We took a big step tonight to earn another 80 minutes of footie. It’s knockout rugby. There is no room for any more mistakes."
But the answer about how to deal with the Pro14 champions might just be in the question.
In much the same fashion as Ireland won the Six Nations, Cullen will recognise now that sticking to the template that has earned seven European wins on the bounce is the way to go.
If Munster’s late win over Toulon was a throwback to their passion-filled glory days, Leinster’s victory over Saracens was a study in professionalism.
Cold, clinical, and, in the end, relatively comfortable.
Leinster are missing a number of senior players - Robbie Henshaw and Sean O'Brien among those - but you’d struggle to notice. The desire is evident too.
"We have a really motivated squad," said Nacewa. "They are really hungry; a lot of guys coming off a Grand Slam and adding another level of intensity to the team.
"And we’re a strong squad, there are a lot of guys who weren’t playing tonight who would have been itching to be there."
Leinster must be wary but more importantly they must not deviate.
The three-time champions are approaching juggernaut mode as the tantalising prospect of an all-Irish final in Bilbao and another star on the jersey gets closer.
Let Scarlets figure it out.
