@article{TAC_2002_10_a1,
     author = {Ronald Brown and Keith A. Hardie and Klaus Heiner Kamps and Timothy Porter},
     title = {A homotopy double groupoid of a {Hausdorff} space},
     journal = {Theory and applications of categories},
     pages = {71--93},
     publisher = {mathdoc},
     volume = {10},
     year = {2002},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/TAC_2002_10_a1/}
}