@article{10_4153_CJM_1970_041_2,
     author = {Hardie, K. A.},
     title = {Homotopy of {Natural} {Transformations}},
     journal = {Canadian journal of mathematics},
     pages = {332--341},
     year = {1970},
     volume = {22},
     number = {2},
     doi = {10.4153/CJM-1970-041-2},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1970-041-2/}
}