( 0, 0 ) - O \hspace{5mm} ( 1, 0 ) - E_1 \hspace{5mm} ( 0, 1 ) - E_2\\ ( a, a+1 ) - O^{'} \hspace{5mm} ( 1, a ) - E_1^{'} \hspace{5mm} ( a, 0 ) - E_2^{'}