theorem
  for f being FinSequence st len f is even holds len f = 2 * Center f - 2
proof
  let f be FinSequence;
  assume ex k being Nat st len f = 2*k;
  hence len f = 2 * (len f div 2) + 2*1 - 2 by NAT_D:18
    .= 2 * Center f - 2;
end;
