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2. E.S. Ljapin, On internal extensions of partial operations to complete associative ones, "Izv. Vysš. Učebn. Zaved. Matematika," No. 7(242), 1982, 40–44 (in Russian).
3. V.V. Rozen, Partial operations in ordered sets, Saratov Univ. Press, 1973. (in Russian)
4. E.S. Ljapin, An abstract characterization of partial groupoids of words with synonyms, "Algebraic Theory of Semigroups. Colloq. Math. Soc. János Bolyai", 20, 1976, 341–356.
5. E.S. Ljapin, Partielle Operationen in der Theorie der Halbgruppen, "Lecture Notes in Mathematics", 855, 1978, 33–48.