theorem Th36a: :: Theorem 3.6 a)
  Tern (a,b,c) = Tern (a,c,b)
  proof
    Tern (a,b,c) = Tern (a,b,Tern(a,c,a`)) by Th35
                .= Tern (Tern(a,b,a),c,Tern(a,b,a`)) by TDis
                .= Tern (a,c,Tern(a,b,a`)) by Th32
                .= Tern (a,c,b) by Th35;
    hence thesis;
  end;
