Donald E. Knuth Frank Ruskey Efficient Coroutine Generation of Constrained Gray Sequences. 183-208 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2635&spage=183 2004 conf/birthday/2004dahl Essays in Memory of Ole-Johan Dahl db/conf/birthday/dahl2004.html#KnuthR04