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 Th95:
  A\iffB in F iff \not\notA\iffB in F
  proof
    \not\notA\impA in F & A\imp\not\notA in F by Th64,Th65; then
    (\not\notA\impA)\and(A\imp\not\notA) in F &
    (\not\notA\impA)\and(A\imp\not\notA)\imp(\not\notA\iffA) in F
    by Th35,Def38;
    then
A1: \not\notA\iffA in F by Def38; then
    A\iff\not\notA in F by Th90;
    hence thesis by A1,Th91;
end;
