The Punch Brothers’ fourth album leaves no doubt: they aren't just newgrass, they're nextgrass. Or more accurately, prog-grass, as exemplified by “Familiarity,” the opening track here. It's an epic, 10-minute suite brimming with bold shifts in tempo, meter, and mood. If it were played on synths and electric guitars instead of banjo and mandolin, it wouldn't sound out of place on an album by vintage Genesis (or at least Radiohead, whom the band covered on their last album). But the PB's still find time to strut their classical chops on a few inspired pieces by Debussy and Scriabin, and they show off their roots cred with a fluid take on the traditional tune "Boll Weevil."