Rederive the Ward-Takahashi identity (12.142) using operator methods. One may assume without loss of generality that the $y_1, y_2, . . y_p$ are already time-ordered. One can then write the T-product as a sum of terms with explicit $\theta$-functions enforcing the time-ordering of the current relative to the $\phi_n$ fields, thereby facilitating the application of the spacetime-derivative. The contact terms arise from the $\mu=0$ derivative, which result in a series of equal-time commutators, at which point (12.128) may be employed.