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PARK CITY, Utah -- The Grateful Dead, or what scarcely remains of them nowadays, are no less powerful when they take the stage -- at least, to those who know them. Maybe moreso, given how fast the sands of time are running out.

And so it was late Sunday night, the eve of the premiere of Long Strange Trip, the nearly four hour documentary executive produced by Martin Scorsese that would soon unspool at the Sundance Film Festival.

SEE ALSO:Four-hour Grateful Dead doc 'Long Strange Trip' headed to Amazon

What was once a thundering ensemble of at least six musicians is now just three old stalwarts: Bob Weir on guitar and vocals, backed by drummer Billy Kreutzmann and percussionist Mickey Hart. Despite their advanced age and depleted numbers, the boys can still shake down a room full of 'Deadheads.

Which is exactly what they did Sunday night at The Spur, a 145-person capacity bar in Park City, Utah, where Sundance is still raging. The tiny, invite-only concert was packed with 'heads, including a healthy contingent of still-in-town celebs like Laura Dern, Kevin Bacon, Woody Harrelson, Nat Faxon and more.

Weir, Kreutzmann and Hart don't go by "The Grateful Dead" anymore, but let's call them what they are.

Remaining members Weir, Kreutzmann and Hart (minus bassist Phil Lesh, who joined them for the last time at the summer 2015 Fare Thee Well finale concerts in Chicago) don't really officially go by "The Grateful Dead" anymore, but seeing as they're the closest thing left in existence, let's call them what they are.

And they're still superb.

Sunday night's mini-show kicked off with a cover -- they've always been a glorified house band, right? -- of the obscure, inquisitive Little Feat song "It's So Easy to Slip," with just Weir, Kreutzmann and Hart, a stripped-down configuration that any 'head would relish seeing these days, for the rarity of it alone.

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This is not a touring trio. This was special.

"We just love to play, man," Kreutzmann said as he washed up in the men's room right before they took the stage. And it was all too apparent; Kreutzmann, the Dead's master timekeeper on a full drum kit since their late-'60s origins, did the entire set with his bare hands on a small cajon drum -- and was none the less into it all.

Bill Kreutzmann on the cajon drum.Credit: mashable/josh dickey

Next, erstwhile Blues Traveler frontman John Popper took the stage, whipped out an A-minor harmonica and filled in on what would've been Jerry Garcia's leads for "West L.A. Fadeaway," a tune Garcia used to love singing.

Popper gave up the stage for a three-man "Loose Lucy," before Harrelson -- who had been jamming in the wings, squatting just below the speakers -- joined on vocals, putting on a convincing Elvis impersonation for "Heartbreak Hotel."

Grand and freewheeling as it all was, this was to be a short performance, nothing like the three- to four-hour musical opuses that the Dead used to deliver.

The Grateful Dead -- plus Woody Harrelson! -- on Sunday night at Sundance.Credit: mashable/josh dickey

So they made it fast with eternal crowd-pleaser "Not Fade Away," Harrelson still on the mic, the vocal refrain of which the small but feisty gathering grabbed up and carried on long after the band waved their goodbyes and left the stage.

And all that was left at this Sundance Film Festival, Grateful Dead-wise, was that 239 minutes of documentary, Long Strange Trip, scheduled to screen on Monday night.

That's definitely long, certainly strange, and guaranteed to be a trip.

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