Can 3DSystems [sue Formlabs](http://www.wired.com/design/2012/11/3d-systems-formlabs-lawsuit/) over [US Patent 5597520](http://patft.uspto.gov/netacgi/nph-Parser?Sect2=PTO1&Sect2=HITOFF&p=1&u=/netahtml/PTO/search-bool.html&r=1&f=G&l=50&d=PALL&RefSrch=yes&Query=PN/5597520)? If it was issued in 1997, wouldn't it have expired in 2007?