reserve i for Nat,
  j for Element of NAT,
  X,Y,x,y,z for set;
reserve C for initialized ConstructorSignature,
  s for SortSymbol of C,
  o for OperSymbol of C,
  c for constructor OperSymbol of C;
reserve a,b for expression of C, an_Adj C;
reserve t, t1,t2 for expression of C, a_Type C;
reserve p for FinSequence of QuasiTerms C;
reserve e for expression of C;
reserve a,a9 for expression of C, an_Adj C;

theorem
  for a being quasi-adjective of C holds Non a <> a
proof
  let a be quasi-adjective of C;
  per cases;
  suppose a is positive;
    then reconsider a9 = a as positive quasi-adjective of C;
    Non a9 is negative quasi-adjective of C;
    hence thesis;
  end;
  suppose a is negative;
    then reconsider a9 = a as negative quasi-adjective of C;
    Non a9 is positive quasi-adjective of C;
    hence thesis;
  end;
end;
