∑j12⁢j342⁢j12+1⁢2⁢j34+1⁢2⁢j13+1⁢2⁢j24+1⁢j1j2j12j3j4j34j13j24j⁢j1j2j12j3j4j34j13′j24′j=δj13,j13′⁢δj24,j24′.