theorem Th28:
  [.a,b".] = [.b,a.] |^ b"
proof
  thus [.a,b".] = a" * b * a * b" .= 1_G * (a" * b * a * b") by GROUP_1:def 4
    .= (b"" * b") * (a" * b * a * b") by GROUP_1:def 5
    .= (b"" * b") * (a" * b * a) * b" by GROUP_1:def 3
    .= b"" * (b" * (a" * b * a)) * b" by GROUP_1:def 3
    .= [.b,a.] |^ b" by Th16;
end;
