reserve i, i1, i2, j, j1, j2, k, m, n, t for Nat,
  D for non empty Subset of TOP-REAL 2,
  E for compact non vertical non horizontal Subset of TOP-REAL 2,
  C for compact connected non vertical non horizontal Subset of TOP-REAL 2,
  G for Go-board,
  p, q, x for Point of TOP-REAL 2,
  r, s for Real;

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;
