theorem Th67:
  for T, A, F holds T-tail F in (Polish-WFF-set(T, A))^^Polish-arity F
proof
  let T, A, F;
  consider n, t, u such that
      A1: t in T and
      A2: n = A.t and
      A3: u in Polish-WFF-set(T, A)^^n and
      A4: F = t^u by Th32;
  T-head F = t & T-tail F = u by A1, A4, Th52;
  hence thesis by A2, A3;
end;
