Matt Mitchell writes music to challenge himself and his fellow musicians, forcing them onto their toes and keeping them there. By his telling, it took years for his quintet, Phalanx Ambassadors, to master the shifting time signatures, oddly overlain harmonies and dyspeptic, misdirected melodies that define its debut album. So what goals should a listener set when taking in something so complex? Follow along as closely as possible, attuned to every ricocheting unison between Patricia Brennan’s vibraphone and Miles Okazaki’s guitar, parsing every mind-defying displacement between Mitchell’s right hand and his left? Or simply revel in the glistening textural play between these instruments (a bassist and drummer round out the group), and the fervor of the improvisers? On “Stretch Goal” — as on the album’s other six tracks — both options work, and there’s nothing mutually exclusive about them. RUSSONELLO