I've pasted a brute force solution that uses Peter Norvig's tree-search function at http://paste.lisp.org/display/44376 -- ******************************************* Chiao-Lun Cheng Van Voorhis Group Department of Chemistry, Room 6-234 Massachusetts Institute of Technology 77 Massachusetts Avenue Cambridge, MA 02139-4307 Email : chiao@mit.edu Tel : +1 (617) 253-1539 Website : http://zeno.unixboxen.net *******************************************