theorem Th71:
  for T, A, F, n st F in Polish-expression-hierarchy(T, A, n+1) holds
      rng Polish-WFF-args F c= Polish-expression-hierarchy(T, A, n)
proof
  let T, A, F, n;
  assume F in Polish-expression-hierarchy(T, A, n+1);
  then reconsider G = F as Element of Polish-expression-hierarchy(T, A, n+1);
  rng Polish-WFF-args F = rng Polish-WFF-args G by Th60, Th26;
  hence thesis by FINSEQ_1:def 4;
end;
