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