Rational "mixing" properties in infinite measure spaces

I'll start with a review of some "rational" ergodic & mixing properties of infinite measure preserving transformations: HK (Hopf-Krickeberg) mixing, rational weak mixing and rational ergodicity. Then I'll show that some group extensions of "Gibbs-Markov semiflows" are HK mixing. This is done via a local limit theorem for these semiflows.