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;

theorem Th66:
  A\iff\not\notA in F
  proof
A1: (A\imp\not\notA)\and(\not\notA\impA)\imp(A\iff\not\notA) in F by Def38;
    A\imp\not\notA in F & \not\notA\impA in F by Th64,Th65;
    then (A\imp\not\notA)\and(\not\notA\impA) in F by Th35;
    hence thesis by A1,Def38;
  end;
