theorem
  L is subst-correct vf-qc-correct implies
  \ex(x,A\orA)\iff\ex(x,A) in G
  proof
    A\orA\impA in G & A\impA\orA in G by Def38,Th52;
    then A\orA\iffA in G by Th43;
    hence thesis by Th129;
  end;
