theorem Th26:
  for P, A, n holds Polish-expression-hierarchy(P, A, n)
      c= Polish-expression-set(P, A)
proof
  let P, A, n;
  set Q = Polish-expression-hierarchy(P, A, n);
  set X = the set of all Polish-expression-hierarchy(P, A, m) where m is Nat;
  let a;
  assume A1: a in Q;
  Q in X;
  hence thesis by A1, TARSKI:def 4;
end;
