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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def min(self, expr, extra_constraints=(), solver=None, model_callback=None):
 """ Return the minimum value of `expr`. :param expr: expression (an AST) 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) """

        if self._solver_required and solver is None:
            raise BackendError("%s requires a solver for evaluation" % self.__class__.__name__)

        return self._min(self.convert(expr), extra_constraints=self.convert_list(extra_constraints), solver=solver, model_callback=model_callback)