I always liked the trees in Richard Jeffrey, ''Formal Logic : Its Scope and Limits''. Jon Awbrey -- inquiry list: http://stderr.org/pipermail/inquiry/ knol: http://knol.google.com/k/-/-/3fkwvf69kridz/1 mof: http://mathoverflow.net/users/1636/jon-awbrey mwb: http://www.mywikibiz.com/Directory:Jon_Awbrey