theorem Th9:
for X be non-empty FinSequence,
    x be set st x in product X holds x is FinSequence
proof
   let X be non-empty FinSequence, x be set;
   assume x in product X; then
   consider g be Function such that
A1: x = g & dom g = dom X
  & for i be object st i in dom X holds g.i in X.i by CARD_3:def 5;
   dom g = Seg len X by A1,FINSEQ_1:def 3;
   hence x is FinSequence by A1,FINSEQ_1:def 2;
end;
