I looked into AliasHelper - it has utility methods to pushdown expressions. i.e. suppose we have f1 filter on top of Project, how to transform f1 so that we can use it below Project. For this it creates mapping between newName -> oldExpression.    In our case, we want different kind of map. given an expression coming from below the project, how will it look above Project. For this we need oldAttributeReference -> newAttributeReference map. 