theorem Th65:
  for T, A holds Polish-WFF-set(T, A) is T-headed
proof
  let T, A, p;
  assume p in Polish-WFF-set(T, A);
  then consider n such that A3: p in Polish-expression-hierarchy(T, A, n+1)
      by Th28;
  set U = Polish-expression-hierarchy(T, A, n);
  p in Polish-expression-layer(T, A, U) by A3, Th23;
  hence thesis by Th21;
end;
