Ewen Denney John Power Konstantinos Tourlas Hiproofs: A Hierarchical Notion of Proof Tree. 341-359 2006 155 Electr. Notes Theor. Comput. Sci. http://dx.doi.org/10.1016/j.entcs.2005.11.063 db/journals/entcs/entcs155.html#DenneyPT06