reserve x for object;
reserve D for set;
reserve p for PartialPredicate of D;
reserve D for non empty set;
reserve p,q,r for PartialPredicate of D;

theorem
  for D being set holds PP_not(PP_True(D)) = PP_False(D)
  proof
    let D be set;
    PP_not(PP_False(D)) = PP_True(D) by Th43;
    hence thesis;
  end;
