Contour integral trick with propagators

pleasehelpmeno
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Hi I am struggling trying to see understand the basic propagator integral trick.

\int \frac{d^{3}p}{(2\pi^{3})}\left\lbrace \frac{1}{2E_{p}}e^{-ip.(x-y)}|_{p_{0}=E_{p}}+\frac{1}{-2E_{p}}e^{ip.(x-y)}|_{p_{0}=-E_{p}}\right\rbrace = \int \frac{d^{3}p}{(2\pi^{3})}\int \frac{dp^{0}}{(i2\pi)}\frac{-1}{p^{2}-m^{2}}e^{-ip.(x-y)}

I know it can be solved by contour integration about poles p^{0}=E_{p} and p^{0}=-E_{p} respectively but I just can't do it. Any help would be appreciated, I can provide working to what I have done but there doesn't seem much point because I am very confused.
 
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Most books race through that part way too quickly, in my opinion. The best derivation I've found online (and the one that finally helped me to understand it) is in http://www.quantumfieldtheory.info/website_Chap03.pdf, around page 70.
 
thanks
 
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