no. 3
There is no complete axiom system for shuffle expressions
RAIRO. Theoretical Informatics and Applications, Tome 33 (1999) no. 3, pp. 271-277 
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     publisher = {EDP-Sciences},
     volume = {33},
     number = {3},
     mrnumber = {1728427},
     zbl = {0951.68066},
     language = {en},
     url = {https://www.numdam.org/item/ITA_1999__33_3_271_0/}
}
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Szepietowski, A. There is no complete axiom system for shuffle expressions. RAIRO. Theoretical Informatics and Applications, Tome 33 (1999) no. 3, pp. 271-277. https://www.numdam.org/item/ITA_1999__33_3_271_0/

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