theorem
  for D being set, D9 being FinSequenceSet of D holds D9 c= D*
proof
  let D be set, D9 be FinSequenceSet of D;
  let a be object;
  assume a in D9;
  then a is FinSequence of D by Def3;
  hence thesis by FINSEQ_1:def 11;
end;
