quantum field theory - Wick's theorem and Feynman propagator

(this is the image from book 'No nonsense QFT' by Jakob Schwichtenberg, page no, 426) The quantity $[\phi_-(x),\phi_+(y)]$ is like an operator inside the bra-kets $\langle 0|$ and $|0\rangle$. I'm

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An Introduction To Quantum Field Theory: Peskin, Michael E

quantum field theory - Wick's theorem and Feynman propagator