Why don’t more ensembles try to unite the spontaneity of free jazz with the discipline of straightahead, as the Greg Burk Quartet does on Carpe Momentum (Soul Note)? Probably because it’s really hard. Nevertheless, pianist/composer Burk has found a way to write pieces that incorporate elements of both, and he and his bandmates Jerry Bergonzi (saxophones), Jonathan Robinson (bass) and Gerald Cleaver (drums) have found a way to play them brilliantly. Check out “Look to the Astroid,” where Burk introduces two thematic elements-one fast, one slow-that seem to expand and contract the piece itself at the whim of the improvisers. Burk says that the time signature acts like gravity in “Burk’s Quirks” and “Hupid Stumid,” in that it has a pull that one can nevertheless resist; the tension adds thrills aplenty to the free-bop daring of the former track (particularly on Bergonzi’s lines) and the wilted harmonies of the latter. Although they do play some straightahead, most people wouldn’t consider the feinting melody on “For George Russell” to be particularly straight; at the opposite extreme is “Ink,” eight minutes of “through-composed rubato” that sounds random at first but becomes mesmerizing. The rest of Carpe Momentum is mesmerizing from the get-go.