Well Quasi Orders and the Shuffle Closure of Finite Sets.

Flavio D'Alessandro Gwénaël Richomme Stefano Varricchio Well Quasi Orders and the Shuffle Closure of Finite Sets. 260-269 2006 conf/dlt/2006 Developments in Language Theory http://dx.doi.org/10.1007/11779148_24 db/conf/dlt/dlt2006.html#DAlessandroRV06