@article{SLC_1985_13_a7,
     author = {Gareth Jones},
     title = {Operations on {Maps} and {Hypermaps}},
     journal = {S\'eminaire lotharingien de combinatoire},
     year = {1985},
     volume = {13},
     url = {http://geodesic.mathdoc.fr/item/SLC_1985_13_a7/}
}