\phi_0 \; (\eta_{ab} \; \zeta^a \; \zeta^b) \; \left\{\eta_{ab} \;[x^a-x_0^a] \; [x^b-x_0^b] - \eta_{ab} \; \zeta^a \; \zeta^b \right\} \over (\eta_{ab} \;[x^a-x_0^a] \; [x^b-x_0^b] - \eta_{ab} \; \zeta^a \; \zeta^b  )^0 + 0 (\eta_{ab} \;[x^a-x_0^a] \; \zeta^b)^0}