1. J. Berman, Distributive lattices with an additional unary operation, Preprint.
2. J. Berman and P. Dwinger, De Morgan algebras: free products and free algebras, Preprint.
3. G. Grätzer, Lattice theory, Freeman and Co., San Francisco (1971).
4. P. Halmos, Lectures on Boolean algebras, Van Nostrand, Princeton (1963).
5. W. Kneale and M. Kneale, The Development of logic, Oxford University Press, Oxford (1962).