Fabio Burderi Giuseppa Castiglione Antonio Restivo Higman's Theorem on Discrete Sets. 435-446 2006 74 Fundam. Inform. 4 http://iospress.metapress.com/openurl.asp?genre=article&issn=0169-2968&volume=74&issue=4&spage=435 db/journals/fuin/fuin74.html#BurderiCR06