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\bibitem[{Piperno \& Tronci}(1990)]{Piperno+Tronci1990} Adolfo Piperno, and Enrico Tronci. "Regular Systems of Equations in $\lambda$-calculus." \textit{Int. J. Found. Comput. Sci.} 1, no. 3 (1990): 325--340. DOI: 10.1142/S0129054190000230.

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