theorem
  for f being sequence of D st q = f|(Seg i) holds len q = i
proof
  let f be sequence of D;
  reconsider i as Element of NAT by ORDINAL1:def 12;
  dom(f|(Seg i)) = Seg i by FUNCT_2:def 1;
  hence thesis by FINSEQ_1:def 3;
end;
