They weren't even done writing it when they went into the studio to record that one, either. It was done pretty quickly.
When I worked at a music store in the mid-'70s, I went downstairs for some reason (the teaching rooms were near the stairway) and I heard sections of '21st Century Schizoid Man' and 'Mirrors' being played repeatedly, so I went to the room where that teacher worked. He was transcribing it.
