theorem Th23:
  for a,b being object, f being FinSequence st n in dom f holds
  (<*a,b*>^f).(n+2) = f.n
  proof
    let a,b be object, f be FinSequence;
    len <*a,b*> = 2 by FINSEQ_1:44;
    hence thesis by FINSEQ_1:def 7;
  end;
