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2. M. R. Bird: Binary relations and flow-diagrams. — Memorandum No.11, Sept. 1969, Computer and Logic Research Group, University of Swansea.
3. R. M. Burstall, P. Landin: Programs and their proofs: an algebraic approach. — Machine Intelligence 4, (B. Meltzer, D. Michie Eds.), Edinburgh University Press 1969.
4. P. M. Cohn: Universal algebra. — Harper Row. New York — London 1965.
5. S. Eilenberg, J. B. Wright: Automata in general algebras. — Information and Control 11, 4, Oct. 1967.