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REFERENCES
| M. V. Lawson, Inverse Semigroups: The Theory of Partial Symmetries. Singapore: World Scientific, 1999.
E. S. Lyapin, Semigroups. Providence, RI: Amer. Math. Soc., 1974.
Weinstein, A. "Groupoids: Unifying Internal and External Symmetry." Not. Amer. Math. Soc. 43, 744-752, 1996.
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