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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 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]