theorem Th68:
  \notA\impB in F iff \notB\impA in F
  proof
A1: \notA\impB in F iff \notB\imp\not\notA in F by Th58;
A2: \notB\impA in F iff \notA\imp\not\notB in F by Th58;
    \not\notA\impA in F & \not\notB\impB in F by Th65;
    hence thesis by A1,A2,Th45;
  end;
