theorem Th2: for f be FinSequence st len f = 0 holds f/^n = f
  proof
    let f be FinSequence;
    assume
A1: len f = 0;
    per cases;
    suppose
A2:   n = 0;
      then len (f/^n) = len f - n by RFINSEQ:def 1
      .= 0 by A1,A2;
      then f/^n = {};
      hence thesis by A1;
    end;
    suppose
      n > 0;
      then f/^n = {} by A1,RFINSEQ:def 1;
      hence thesis by A1;
    end;
  end;
