
theorem XL1:
  for f be XFinSequence, g be FinSequence holds
    len (f^g) = len f + len g & len (g^f) = len f + len g
  proof
    let f be XFinSequence, g be FinSequence;
    Seg (len f + len g) = dom (g^f) by Def2
    .= Seg (len (g^f)) by FINSEQ_1:def 3;
    hence thesis by Def1,FINSEQ_1:6;
  end;
