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Method for selectively restricting access to computer systems. Filed on Apr 13, 1998 and granted on Feb 27, 2001. Available at http://www.google.com/patents/US6195698 ↑ Template:Cite journal ↑ 16.0 16.1 Template:Cite journal ↑ 17.0 17.1 von Ahn, L; M.