Dark, droning, and dense, this 1973 collaboration between King Crimson guitarist Robert Fripp and Roxy Music keyboardist Brian Eno was not only something of a slap in the face to the prog rock excesses of the time but also a gesture in the direction of classical minimalism. It consists of two multi-track suites: “The Heavenly Music Corporation” (in five parts) and “Swastika Girls” (in two parts). All feature a variation on what would come to be called “Frippertronics” — a loop-based approach whereby his guitar is recorded and layered on tape, at which point he plays solos over the resulting bed of shifting chords.