no. 1
Free extensions of double categories
Cahiers de Topologie et Géométrie Différentielle Catégoriques, Tome 45 (2004) no. 1, pp. 35-80. 
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     author = {Dawson, R. J. M. and Pare, R. and Pronk, D. A.},
     title = {Free extensions of double categories},
     journal = {Cahiers de Topologie et G\'eom\'etrie Diff\'erentielle Cat\'egoriques},
     pages = {35--80},
     publisher = {Dunod \'editeur, publi\'e avec le concours du CNRS},
     volume = {45},
     number = {1},
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     mrnumber = {2040662},
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     url = {http://www.numdam.org/item/CTGDC_2004__45_1_35_0/}
}
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Dawson, R. J. M.; Pare, R.; Pronk, D. A. Free extensions of double categories. Cahiers de Topologie et Géométrie Différentielle Catégoriques, Tome 45 (2004) no. 1, pp. 35-80. http://www.numdam.org/item/CTGDC_2004__45_1_35_0/

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