In the case of ⟨ ⟨ σ σ 4 ⟩ ⟩ , fusion rules allow two primary fields in all channels: the identity field and the energy field. In this case we write the conformal blocks in the case ( z 1 , z 2 , z 3 , z 4 ) = ( x , 0 , ∞ ∞ , 1 ) only: the general case is obtained by inserting the prefactor x 1 24 ( 1 − − x ) 1 24 ∏ ∏ 1 ≤ ≤ i < j ≤ ≤ 4 z i j − − 1 24 , and identifying x with the cross-ratio.