theorem
  [.a |^ n,b.] = a |^ (- n) * ((a |^ b) |^ n)
proof
  thus [.a |^ n,b.] = (a |^ n)" * (b" * (a |^ n) * b) by Th16
    .= a |^ (- n) * ((a |^ n) |^ b) by GROUP_1:36
    .= a |^ (- n) * ((a |^ b) |^ n) by GROUP_3:27;
end;
