theorem
  for p,q being FinSequence st p = p^q or p = q^p holds q = {}
proof
  let p,q be FinSequence such that
A1: p = p^q or p = q^p;
   len(p^q) = len p + len q & len(q^p) = len q + len p by Th22;
  hence thesis by A1;
end;
