theorem Th36c: :: Theorem 3.6 c)
  Tern (a,b,c) = Tern (c,b,a)
  proof
    Tern (a,b,c) = Tern (a,c,b) by Th36a
                .= Tern (c,b,a) by Th36b;
    hence thesis;
  end;
