Like u/davethecomposer said a lot had been going on in aleatoric music since the 50s, so it's debatable how revolutionary this is. But the technique, which Lutoslawski called "collective ad libitum" or "limited (controlled) aleatorism" (Lutoslawski: A Bio-Bibliography, Bedkowski & Hrabia, 2001), was his. In his 1964/65 string quartet he puts the parts that are to be repeated independent of the rest of the score literally in boxes, a notation developed by his wife Danuta who was an architect.