see patent #6,496,154 http://patimg2.uspto.gov/.piw?Docid=06496154&homeurl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO1%2526Sect2%3DHITOFF%2526d%3DPALL%2526p%3D1%2526u%3D%2Fnetahtml%2Fsrchnum.htm%2526r%3D1%2526f%3DG%2526l%3D50%2526s1%3D6,496,154.WKU.%2526OS%3DPN%2F6,496,154%2526RS%3DPN%2F6,496,154&PageNum=&Rtype=&SectionNum=&idkey=C2F585DEB965