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 Th56:
  A\imp(\notA\impB) in F
  proof
    A\and\notA\impB\imp(A\imp(\notA\impB)) in F & A\and\notA\impB in F
    by Th47,Def38;
    hence thesis by Def38;
  end;
