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

theorem Th21:
  for f being FinSequence holds S_Drop(len f-'1,f)=len f-'1
proof
  let f be FinSequence;
  (len f-'1) mod (len f-'1)=0 by NAT_D:25;
  hence thesis by Def1;
end;
