θ⁡(z+∫P1P3ω|Ω)⁢θ⁡(z+∫P2P4ω|Ω)⁢E⁢(P3,P2)⁢E⁢(P1,P4)+θ⁡(z+∫P2P3ω|Ω)⁢θ⁡(z+∫P1P4ω|Ω)⁢E⁢(P3,P1)⁢E⁢(P4,P2)=θ⁡(z|Ω)⁢θ⁡(z+∫P1P3ω+∫P2P4ω|Ω)⁢E⁢(P1,P2)⁢E⁢(P3,P4),