  We present a complete set of multiparticle correlation observables for
ultrarelativistic heavy-ion collisions. These include moments of the
distribution of the anisotropic flow in a single harmonic, and also mixed
moments, which contain the information on correlations between event planes of
different harmonics. We explain how all these moments can be measured using
just two symmetric subevents separated by a rapidity gap. This presents a
multi-pronged probe of the physics of flow fluctuations. For instance, it
allows to test the hypothesis that event-plane correlations are generated by
non-linear hydrodynamic response. We illustrate the method with simulations of
events in A MultiPhase Transport (AMPT) model.
