reserve X,Y for set, x,y,z for object, i,j,n for natural number;
reserve
  n for non empty Nat,
  S for non empty non void n PC-correct PCLangSignature,
  L for language MSAlgebra over S,
  F for PC-theory of L,
  A,B,C,D for Formula of L;
reserve
  J for non empty non void Signature,
  T for non-empty MSAlgebra over J,
  X for non empty-yielding GeneratorSet of T,
  S1 for J-extension non empty non void n PC-correct QC-correct
  QCLangSignature over Union X,
  L for non-empty Language of X extended_by ({},the carrier of S1), S1,
  G for QC-theory of L,
  A,B,C,D for Formula of L;
reserve x,y,z for Element of Union X;
reserve x0,y0,z0 for Element of Union (X extended_by ({},the carrier of S1));

theorem
       L is subst-correct vf-qc-correct implies
  \for(x,A)\iff\for(x,\not\notA) in G
  proof assume
A1: L is subst-correct vf-qc-correct;
    A\imp\not\notA in G & \not\notA\impA in G by Th65,Th64;
    then \for(x,A)\imp\for(x,\not\notA) in G &
    \for(x,\not\notA)\imp\for(x,A) in G by A1,Th115;
    then
A2: (\for(x,A)\imp\for(x,\not\notA))\and
    (\for(x,\not\notA)\imp\for(x,A)) in G by Th35;
    (\for(x,A)\imp\for(x,\not\notA))\and
    (\for(x,\not\notA)\imp\for(x,A))\imp(\for(x,A)\iff\for(x,\not\notA)) in G
    by Def38;
    hence thesis by A2,Def38;
  end;
