Given the musical incoherence that plagues many jam bands, the nom de jam Um does not inspire a whole lot of confidence. Thankfully, Um’s album Stray Dog sounds a lot more articulate than their name does. All of Um’s members, including special guest John Medeski, have a sure enough grasp of melodic and harmonic structure that they’re not afraid to take unpredictable angles on the music, and the results are almost always intriguing. The album’s styles range from the sci-fi desolation of “Repulse Impulse,” which could fit into a modern classical program, to motive-driven numbers like “Walter’s World” to straight-up theme-and-improv like the opening “Bushwacker.” Medeski’s imaginative, energetic organ work and bassist Dave Zinno’s awesome talent for keeping everything under control and outside the lines deserve commendation. But the real star here is Hal Crook on the “trom-o-tizer”-a digital-effects altered trombone-an instrument that sounds like an entire brass section, albeit a continuously astringent one. When Crook chases himself around on the furtive, obsessive melody of “Pervert,” you’ll wonder for a moment why anyone ever plays anything else.