http://lilt.ilstu.edu/jhreid/grammar/pcimp.htm
http://french.about.com/library/weekly/aa060799.htm
http://www.laits.utexas.edu/fi/tv/index.php?mode=q&ch=9&ex=5
http://fog.ccsf.cc.ca.us/~creitan/clark.htm
http://www.swarthmore.edu/Humanities/gmoskos1/interexercice/pcompose.htm
http://stronaofrancji.host.sk/j4.php