theorem Th10:
  1 <= len (fin^<*b*>) & len (fin^<*b*>) in dom (fin^<*b*>)
proof
  len (fin^<*b*>) = len fin + 1 by FINSEQ_2:16;
  then 1 <= len (fin^<*b*>) by NAT_1:11;
  hence thesis by FINSEQ_3:25;
end;
