SymbiYosys (sby) is a front-end driver program for Yosys-based formal hardware verification flows. See http://symbiyosys.readthedocs.io/ for documentation on how to use SymbiYosys.
Many example designs using SymbiYosys have been published on the ZipCPU blog. Please consider browsing the formal verification page of the ZipCPU blog for examples and commentary.