1. Inventor's Cert. (Bulgaria) No. 12688, Ref. Zh. Khim., No. 3, No. 3 N472P (1972).
2. Inventor's Cert. (Bulgaria) No. 28735, Ibid., No. 14, No. 14 0262P (1983).
3. Inventor's Cert. (Bulgaria) No. 30848, Ibid., No. 6, No 6 016P (1983)
4. Inventor's Cert. (USSR) No. 325756, Ibid., No. 20, No. 20 0224P (1983).
5. Inventor's Cert. (USSR) No. 761474, Ibid., No. 13, No, 13 0103P (1981).