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\bibcite{doi:10.1002/j.1538-7305.1961.tb03975.x}{1}
\bibcite{berger1966undecidability}{2}
\bibcite{Robinson1971}{3}
\bibcite{Grunbaum:1986:TP:19304}{4}
\bibcite{Kari:1996:SAS:245761.245817}{5}
\bibcite{DBLP:journals/corr/JeandelR15}{6}
