This is not a free improv session. Half the compositions are by Mitchell, and half by Braxton; you can tell which ones are which from the titles, before you even start listening. “Five Twenty One Equals Eight” and “Seven Behind Nine Ninety Seven Sixteen Or Seven,” Mitchell; “Composition 40Q,” “Composition 74B,” “Composition 74A,” all with inscrutable accompanying diagrams, Braxton. Mitchell’s pieces are mostly meditative, unfolding slowly while exploring the timbral extremes he favors (flute paired with bass saxophone, for example). Braxton’s feature pulsing, tootling melodies with a lot of unison playing and close harmony. Sometimes it sounds like chamber music, other times like they’re testing the various horns’ range and fluidity of action, but it all works as music and reveals two extraordinarily keen compositional minds at work, each man challenging the other and rising to the challenges presented.