theorem Th16:
  (h * g)" = g" * h"
proof
  (g" * h") * (h * g) = g" * h" * h * g by Def3
    .= g" * (h" * h) * g by Def3
    .= g" * 1_G * g by Def5
    .= g" * g by Def4
    .= 1_G by Def5;
  hence thesis by Th11;
end;
