FCLoopGetEtaSigns[exp] is an auxiliary function that extracts the signs of i \eta from propagators present in the input expression. The result is returned as a list, e.g. {}, {1}, {-1} or {-1,1}.
This is useful if one wants ensure that all propagators of the given integral or topology follow a particular i \eta sign convention.
Overview, FCTopology, FCLoopSwitchEtaSign.
FAD[{p, m}]
FCLoopGetEtaSigns[%]\frac{1}{p^2-m^2}
\{1\}
SFAD[{p, m^2}]
FCLoopGetEtaSigns[%]\frac{1}{(p^2-m^2+i \eta )}
\{1\}
SFAD[{I p, -m^2}, EtaSign -> -1]
FCLoopGetEtaSigns[%]\frac{1}{(-p^2+m^2-i \eta )}
\{-1\}
CFAD[{p, m^2}]
FCLoopGetEtaSigns[%]\frac{1}{(p^2+m^2-i \eta )}
\{-1\}