reserve x, x1, x2, y, X, D for set,
  i, j, k, l, m, n, N for Nat,
  p, q for XFinSequence of NAT,
  q9 for XFinSequence,
  pd, qd for XFinSequence of D;

theorem Th33:
  card Domin_0(2*n,n) = ((2*n) choose n) / (n+1)
proof
  card Domin_0(2*n,n) = (2*n+1-2*n)/(2*n+1-n)*((2*n) choose n) by Th29
    .=(1*((2*n) choose n))/(n+1) by XCMPLX_1:74;
  hence thesis;
end;
