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;
