hi_class: Horndeski in the Cosmic Linear Anisotropy Solving System

We present the public version of hi cl a ss (www . hiclass-code . net), an extension of the Boltzmann code CLASS to a broad ensemble of modifications to general relativity. In particular, hi cl a ss can calculate predictions for models based on Horndeski's theory, which is the most general scalar-tensor theory described by second-order equations of motion and encompasses any perfect-fluid dark energy, quintessence, Brans-Dicke, f( R) and covariant Galileon models. hi-class has been thoroughly tested and can be readily used to understand the impact of alternative theories of gravity on linear structure formation as well as for cosmological parameter extraction.