@article{TAC_2014_29_a12,
     author = {Anders Kock},
     title = {Projective lines as groupoids with projection structure},
     journal = {Theory and applications of categories},
     pages = {371--388},
     publisher = {mathdoc},
     volume = {29},
     year = {2014},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/TAC_2014_29_a12/}
}