reserve n for Nat;

theorem Th28:
  for f be unfolded FinSequence of TOP-REAL 2 for i,j be Nat
   holds mid(f,i,j) is unfolded
proof
  let f be unfolded FinSequence of TOP-REAL 2;
  let i,j be Nat;
  per cases;
  suppose
    i <= j;
    then mid(f,i,j) = (f/^(i-'1))|(j-'i+1) by FINSEQ_6:def 3;
    hence thesis;
  end;
  suppose
    j < i;
    then mid(f,j,i) = (f/^(j-'1))|(i-'j+1) by FINSEQ_6:def 3;
    then Rev mid(f,i,j) is unfolded by FINSEQ_6:196;
    then Rev Rev mid(f,i,j) is unfolded by SPPOL_2:28;
    hence thesis;
  end;
end;
