Hi, I am stuck on a step Peskin & Schroeder give in their proof of Wick's Theorem (Intro to Quantum Field Theory, p 90). In the middle of the page when they consider the term with no contraction, it seems like in between the 1st and 2nd lines they somehow factor out the normal ordering operator. How is this legal? I've attached a bmp of what I'm talking about. Thanks *Typo...There shouldn't be an equal sign after the last commutator in the bmp.