@article{TAC_2004_12_a7,
     author = {Michael Barr},
     title = {On subgroups of the {Lambek} pregroup},
     journal = {Theory and applications of categories},
     pages = {262--269},
     year = {2004},
     volume = {12},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/TAC_2004_12_a7/}
}