Eval the ast, replacing symbols by their last value in the model.

def eval_ast(self, ast):
        """Eval the ast, replacing symbols by their last value in the model.
        """
        # If there was no last value, it was not constrained, so we can use
        # anything.
        new_ast = ast.replace_dict(self.replacements, leaf_operation=self._leaf_op)
        return backends.concrete.eval(new_ast, 1)[0]