1. On adding relations to homotopy groups;Whitehead;Ann. of Math. (2),1941
2. On the 3-type of a complex;MacLane;Proc. Natl. Acad. Sci. USA,1950
3. Nonabelian Algebraic Topology. Filtered Spaces, Crossed Complexes, Cubical Homotopy Groupoids. With Contributions by Christopher D. Wensley and Sergei V. Soloviev;Brown,2011
4. Combinatorial Homotopy and 4-dimensional Complexes;Baues,1991
5. Double groupoids and crossed modules;Brown;Cah. Topologie Géom. Différ. Catég.,1976