theorem Th1:
  for f being FinSequence st len f > 0 & n > 0 holds len (f|n) > 0
  proof
    let f be FinSequence;
    assume that
A1: len f > 0 and
A2: n > 0;
    per cases;
    suppose
      n <= len f;
      hence thesis by FINSEQ_1:59,A2;
    end;
    suppose
      n > len f;
      hence thesis by FINSEQ_1:58,A1;
    end;
  end;
