Equation Icon  2 ∫ ¯κ-- 4 ′4 ′∘ ----′---′- ret ′ efgh ′ ′ ret ′ ⟨hab(x)hcd(y )⟩ind = N d xd y g (x )g(y )Gabef(x,x )N (x ,y )G cdgh(y, y), (156 )