def _check_satisfiability(self, extra_constraints=(), solver=None, model_callback=None):
return '<STR_LIT>' if self.satisfiable(extra_constraints=extra_constraints, solver=solver, model_callback=model_callback) else '<STR_LIT>'<EOL>
This function does a constraint check and returns the solvers state

:param solver:              The backend solver object.
:param extra_constraints:   Extra constraints (as ASTs) to add to s for this solve
:param model_callback:      a function that will be executed with recovered models (if any)
:return:                    'SAT', 'UNSAT', or 'UNKNOWN'