It is worthwhile to point out that, the supplementary equation Φ Φ i j = 2 ϕ ϕ i ϕ ϕ j ¯ ¯ is only valid for electromagnetic fields; for example, in the case of Yang-Mills fields there will be Φ Φ i j = Tr ( ϝ ϝ i ϝ ϝ ¯ ¯ j ) where ϝ ϝ i ( i ∈ ∈ { 0 , 1 , 2 } ) are Yang-Mills-NP scalars.