theorem Th43:
  A,A9 Is p implies A c= Plane(A,A9) & A9 c= Plane(A,A9) &
  Plane(A,A9) = Plane(A9,A)
  proof
    assume A,A9 Is p;
    then A c= Plane(A9,A) & A9 c= Plane(A,A9) &
    Plane(A,A9) = Plane(A9,A) by Th39,Th40,Th42;
    hence thesis;
  end;
