reserve A,B,p,q,r,s for Element of LTLB_WFF,
  i,j,k,n for Element of NAT,
  X for Subset of LTLB_WFF,
  f,f1 for FinSequence of LTLB_WFF,
  g for Function of LTLB_WFF,BOOLEAN;

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;
