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\bibitem[{Piperno \& Tronci}(1989)]{Piperno+Tronci1989} Adolfo Piperno, and Enrico Tronci. "Regular Systems of Equations in $\lambda$-calculus." In \textit{Ictcs}. Mantova - Italy, 1989. DOI: 10.1142/S0129054190000230.

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