When you want to perform Karlheinz Stockhausen’s “Gruppen” in the Tate Modern, and you really, really, don’t want the three orchestras to fall out of sync, you have to take rather a lot of meetings. The three conductors: “Pencils in hand, they laid out their scores on a table and mimicked their orchestras’ parts in a cacophony of hummed notes, whoops, grunts, bleats and birdlike sounds — and every once in a while, in unison, a triumphant’Bang!'”