* http://www.valand.gu.se/dm/sites04/cm_node27.html
* http://www.valand.gu.se/dm/students/marco/projekt/doc.html
* http://www.gp.se/gp/road/Classic/shared/printArticle.jsp?d=110&a=171083
* http://www.coneculta.gob.mx/artesplasticas.htm