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
  A\orB\imp(\notB\impA) in F
  proof
    A\orB\impB\orA in F & B\orA\imp(\notB\impA) in F by Th36,Th62;
    hence A\orB\imp(\notB\impA) in F by Th45;
  end;
