reserve x,y for set,
  n,m for Nat,
  r,s for Real;
reserve f, g for Function;

theorem
  for f being FinSequence holds f /^ len f = {}
proof
  let f be FinSequence;
  len (f /^ len f) = len f - len f by Def1
    .= 0;
  hence thesis;
end;
