1. R.B. Seligman, U.S. Patent 2 879 298 (Mar. 24, 1959) (to Phillip Morris Inc.).
2. Y. Mizutani, A. Izumi and S. Watanabe, Japan Patent 70 28 996 (Sep. 21, 1970) (to Tokuyama Soda Co. Ltd).
3. S. Yamada, Japan Patent 18 969 (Oct. 11, 1961) (to Nippon Sekiyu Co. Ltd).
4. L.V. Robbins, Jr. and W.J. Porter, Jr., U.S. Patent 3 361 828 (Jan. 2, 1968) (to Esso Research and Engineering Co.).