reserve i, i1, i2, j, k for Nat,
  r, s for Real;
reserve D for non empty set,
  f1 for FinSequence of D;

theorem Th22:
  for n being Nat, f being FinSequence st
  1<=n & n<=len f-'1 holds S_Drop(n,f)=n
proof
  let n be Nat,f be FinSequence;
  assume that
A1: 1<=n and
A2: n<=len f-'1;
  per cases by A2,XXREAL_0:1;
  suppose
    n<len f-'1;
    then n mod (len f-'1)=n by NAT_D:24;
    hence thesis by A1,Def1;
  end;
  suppose
    n=len f-'1;
    hence thesis by Th21;
  end;
end;
