hi_class: Horndeski in the Cosmic Linear Anisotropy Solving System
  We present the public version of hi_class (www.hiclass-code.net), an
extension of the Boltzmann code CLASS to a broad ensemble of modifications to
general relativity. In particular, hi_class 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.
