theorem Th30:
  card Domin_0(2+k,1) = k+1
proof
  card Domin_0(2+k,1)=(2+k+1-2*1)/(2+k+1-1)*((2+k) choose 1) by Th29,NAT_1:11
    .=(k+1)/(2+k)*(2+k) by STIRL2_1:51
    .=k+1 by XCMPLX_1:87;
  hence thesis;
end;
