theorem
  [.a |^ i,b.] = a |^ (- i) * ((a |^ b) |^ i)
proof
  thus [.a |^ i,b.] = (a |^ i)" * (b" * (a |^ i) * b) by Th16
    .= a |^ (- i) * ((a |^ i) |^ b) by GROUP_1:36
    .= a |^ (- i) * ((a |^ b) |^ i) by GROUP_3:28;
end;
