1. M.B. Simmonds, US Patent 6,046,585 (1997).
2. P.I. Nikitin, P.M. Vetoshko, US Patent 20030027197A1 (2003).
3. R.E. Sager, M.B. Simmonds, J.H. Diederichs, K.C. Jensen, R.C. Black, US Patent 6,518,747 (2003).
4. M.C. Tondra, J.M. Anderson, US Patent 6,743,639 (2004).