def _min(self, expr, extra_constraints=(), solver=None, model_callback=None): 
raise BackendError("<STR_LIT>")<EOL>
Return the minimum value of expr.

:param expr: expression (backend object) to evaluate
:param solver: a solver object, native to the backend, to assist in
               the evaluation (for example, a z3.Solver)
:param extra_constraints: extra constraints (as ASTs) to add to the solver for this solve
:param model_callback:      a function that will be executed with recovered models (if any)
:return: the minimum possible value of expr (backend object)