theorem Th90:
  for D be set, z being FinSequence of D holds
  z is Element of (len z)-tuples_on D
proof
  let D be set, z be FinSequence of D;
  z is Element of D* by FINSEQ_1:def 11;
  then z in { z9 where z9 is Element of D*: len z9 = len z};
  hence thesis;
end;
