def _satisfiable(self, extra_constraints=(), solver=None, model_callback=None): 
raise BackendError("<STR_LIT>")<EOL>
This function does a constraint check and returns a model for a solver.

:param solver:              The backend solver object
:param extra_constraints:   Extra constraints (backend objects) to add to s for this solve
:param model_callback:      a function that will be executed with recovered models (if any)
:return:                    True if sat, otherwise false