@article{ITA_2006__40_3_407_0,
     author = {D'Alessandro, Flavio and Varricchio, Stefano},
     title = {Well quasi-orders, unavoidable sets, and derivation systems},
     journal = {RAIRO - Theoretical Informatics and Applications - Informatique Th\'eorique et Applications},
     pages = {407--426},
     publisher = {EDP-Sciences},
     volume = {40},
     number = {3},
     year = {2006},
     doi = {10.1051/ita:2006019},
     zbl = {1110.68060},
     language = {en},
     url = {http://www.numdam.org/articles/10.1051/ita:2006019/}
}