1. The problem statement, all variables and given/known data I am a rookie to the QFT extension in Mathematica called FeynCalc, and tried to use that into solving some quiz. Soon I met a problem upon some condition presented in a problem which declares an relation of two same tensor with different indices results in some value when commuted. I don't know how to express such relationship nor how intelligent the program is in dealing with that. 2. Relevant equations Thai problem is Peskin&Schroeder 3.1. 3. The attempt at a solution Searched the document and examples, no practical suggestions found yet.