http://6moo7.com/vb/showthread.php?s=6345ecf7cd72a28ece594facd514643a&t=9618
http://www.aoua.com/vb/archive/index.php/t-25581.html
http://www.hesab.net/vb/printthread.php?t=1610&page=2&pp=15
http://mathsrv.ku-eichstaett.de/MGF/homes/grothmann/java/zirkel/doc_en/Documentation/index.html