SONG computes the non-linear evolution of the Universe in order to predict cosmological observables such as the bispectrum of the Cosmic Microwave Background (CMB). More precisely, it is a second-order Boltzmann code, as it solves the Einstein and Boltzmann equations up to second order in the cosmological perturbations.
One or more of the following: https://ui.adsabs.harvard.edu/abs/2013JCAP...04..003P , https://ui.adsabs.harvard.edu/abs/2016ibcm.book.....P , and/or https://ui.adsabs.harvard.edu/abs/2014PhRvD..90j3010P