Peter Braun Frank Marschall Transforming Object Oriented Models with BOTL. 2003 72 Electr. Notes Theor. Comput. Sci. 3 http://www1.elsevier.com/gej-ng/31/29/23/127/48/show/Products/notes/index.htt#010 db/journals/entcs/entcs72.html#BraunM03