reserve n,k,b for Nat, i for Integer;

theorem Th3:
  for f being XFinSequence holds mid(f,2,len f)=f/^1
  proof
    let f be XFinSequence;
    A1: 2-1>=0;
    thus mid(f,2,len f) = f/^1 by A1,XREAL_0:def 2;
  end;
