Karl J. Lieberherr Boaz Patt-Shamir Doug Orleans Traversals of object structures: Specification and Efficient Implementation. 370-412 2004 26 ACM Trans. Program. Lang. Syst. 2 http://doi.acm.org/10.1145/973097.973102 db/journals/toplas/toplas26.html#LieberherrPO04