theorem Th66:
  for T, A, n holds Polish-expression-hierarchy(T, A, n) is T-headed
proof
  let T, A, n;
  reconsider U = Polish-expression-hierarchy(T, A, n)
      as Subset of Polish-WFF-set(T, A) by Th26;
  Polish-WFF-set(T, A) is T-headed by Th65;
  then U is T-headed;
  hence thesis;
end;
