Miles Davis’s first real step into “fusion” — or, more accurately, post-jazz — consists of two side-long tracks, each of which ends with a copy ’n’ paste of its opening section. Davis and Wayne Shorter solo a little bit over the endless train-track groove, which comes courtesy of Tony Williams playing nothing but hi-hat. Behind/beneath, Dave Holland’s bass chugs and bounces; three electric pianos create a shimmering ocean of sound; and John McLaughlin underplays for maybe the only time in his entire career.