I have further expanded the explicit computations in that section, now also of the causal propagator. I am copying this over to *causal propagator*, too.

This means there is now much verbatim overlap between the entries *causal propagator* and *advanced and retarded propagators*. It might make sense to just merge the two entries to one. But for the moment I keep them distinct.

This needed some adjustment and fixing. I added in more precise background on the definition, and then an improved writeup of the computation: here.

]]>added to *advanced and retarded propagator* statement and proof of the expression