<http://www.w3.org/1999/02/22-rdf-syntax-ns#first>
<http://ichef-1.bbci.co.uk/news/ws/560/amz/worldservice/live/assets/images/2014/07/16/140716142916_bombillos_en_el_aire_624x351_spl.jpg>
<http://www.bbc.com/mundo/noticias/2014/07/140714_elementos_tungsteno_wolframio_finde_dv.shtml>